Hundred is a numerical puzzle that involves strategically placing digits in a grid to achieve specific sums in each row and column. The puzzle is played on a 3x3 or 4x4 grid. Here’s an example Hundred puzzle:

As you can see, the cells are already filled with digits. Your task is to add extra digits into some or all cells in such a way the the sum of the numbers in each row and each column equals 100.

You can add digits before or after those already in a cell. So for example a 7 could become 17 by adding 1 in front of it, or 71 by putting the 1 after it.

There are no restrictions on how often you can use digits, and all the digits from 0 to 9 are on the table.

Here’s what the example puzzle looks like once it’s been solved:

Eulero, also known as a *Graeco-Latin Square* or *Euler Square*, is a fascinating logic puzzle that combines elements of Latin squares and Sudoku. Here’s what a small puzzle looks like:

The puzzle is played on a square grid. Bigger grids mean harder puzzles. Each cell in the grid must contain one letter and one digit. Normally, the letters and digits corresponds to the size of the grid. So in a 5x5 grid, we use use the letters A to E and the digits 1 to 5.

The objective of Eulero is to fill out the grid such that every row and every column contains each letter and each digit exactly once - ie no row or column can have a repeated letter or digit. Additionally, no two cells in the grid can contain the same pairing of a letter and a digit.

Here’s what our example puzzle looks like when complete:

**Start with Known Pairs.** Some cells are already filled in, or are partially filled in (depending on the difficulty level of the puzzle), so use them as a starting point. The given pairs can help you determine what can't be in the same row or column.

**Elimination. ** If you're unsure about where a particular symbol should go, consider where it can't go. This process of elimination can narrow down your options.

**Sudoku.** Eulero follows similar rules to Sudoku, which means that many Sudoku techniques can either be used directly, or adapted, to help solve the puzzle. See our detailed three-part Sudoku tutorial here for some ideas.

Fancy filling some thermometers? We publish this puzzle occasionally in our free Puzzle Weekly magazine. You should totally sign up for that if you haven’t already, as it puts 28 brand new puzzles in your inbox every week.

You can also find lots of Thermometers puzzles in our *Jumbo Adult Puzzle Book* – which happens to include more than *500 puzzles* of 20 different varieties.

Stitches is an intriguing logic puzzle that involves connecting different regions of a grid with lines. This is what a small grid looks like:

The puzzle is played on a square grid divided into variously shaped regions. Each region must be connected to every one of its neighbouring regions by exactly one line. These lines are referred to as "stitches."

A stitch is a line that spans one cell, connecting two orthogonally adjacent cells from different regions.

A cell can be traversed by at most one stitch.

Numbers along the edge of the grid indicate how many line endpoints must be placed in the corresponding row or column.

Here is what the above example looks like once it’s been completed:

**Start with Edge Clues.** Look at the rows and columns with numbers on the grid's edge. This tells you how many times a stitch must end in that row or column, guiding where to draw your initial stitches.

**Use the Single Cell Rule.** Since a cell can only have one stitch, if a stitch already passes through a cell, you can’t draw another stitch through that cell.

**Check for Isolated Regions.** Be wary of creating isolated regions where a region cannot possibly connect to a neighbour because all potential connecting cells are used up.

**Consider Stitch Length.** Stitches only extend one cell. This limits the possible connections, especially near the grid's edges or in tightly packed areas.

**Balance Edge Requirements.** Continuously cross-check the stitches with the edge numbers. Each row and column should have the exact number of endpoints as indicated by the clues.

Stitches is a great puzzle for those who enjoy spatial reasoning and planning. The challenge lies in ensuring all regions are properly connected while adhering to the grid's constraints.

Fancy filling some thermometers? We publish this puzzle occasionally in our free Puzzle Weekly magazine. You should totally sign up for that if you haven’t already, as it puts 28 brand new puzzles in your inbox every week.

You can also find lots of Thermometers puzzles in our *Jumbo Adult Puzzle Book* – which happens to include more than *500 puzzles* of 20 different varieties.

Thermometers is a pure logic puzzle played on a square or rectangular grid filled with thermometer shapes. Each thermometer has a base (the bulbous end) and a top. In simple puzzles, thermometers may be placed horizontally and / or vertically. In more difficult levels, the thermometers may be ‘broken’ such that they span more than one column or row.

The objective of the game is to fill the thermometers sufficiently that the number of cells filled in a row and column of the grid corresponds to the numbers on the outside of the grid.

Here’s a small example Thermometers puzzle:

- Thermometers can be entirely unfilled, partially filled, or completely filled.
- Thermometer always fill from the base toward the top. This is irrespective of the thermometer's actual orientation on the grid.
- Each filled segment of a thermometer counts as one filled cell.

Here is what the example puzzle looks like when it has been solved:

**Start with Extremes**. Look for rows or columns with a 0 written outside. This means no cell in that row or column is filled, so all thermometers crossing it remain unfilled beyond that cell. Similarly, if a row or column's clue equals its length (e.g., a clue of '5' for a row of 5 cells), then every cell in that row or column is filled.

**Look for Forced Fills**. If a thermometer segment in a row or column is filled, then all segments below it (toward the base) must also be filled. Conversely, if a segment is unfilled, all segments above it (toward the top) must be unfilled too.

** Use Partial Information**. Even if you can't completely determine the fill status of a row, column, or thermometer, partial fills can help. For instance, if you have a row of 8 cells with a clue of '6', and two thermometers with bases in that row, you know at least some segments of those thermometers must be filled to meet the clue.

**Mind the Gaps**. If filling a thermometer segment would exceed the clue number for a row or column, then that segment (and those above it) must remain unfilled.

**Use Pencil Marks**. For cells you’re not sure about, mark potential fills lightly. If they lead to contradictions, you can erase and reassess.

Fancy filling some thermometers? We publish this puzzle occasionally in our free Puzzle Weekly magazine. You should totally sign up for that if you haven’t already, as it puts 28 brand new puzzles in your inbox every week.

You can also find lots of Thermometers puzzles in our *Jumbo Adult Puzzle Book* – which happens to include more than *500 puzzles* of 20 different varieties.

Star Battle is a pure logic puzzle that takes seconds to learn, but can become surprisingly tricky as the difficulty level increases.

The puzzle is played on a square grid which is divided into various regions, delimited by bold lines.

Here is an example of a Star Battle grid:

The objective of the puzzle is to place stars into the cells such that every row, column, and grid contains exactly the same number of stars. Stars cannot be placed in adjacent cells (ie stars cannot be in cells that touch horizontally, vertically, or diagonally). Every region must contain one star when the puzzle is complete.

Here’s what the earlier example grid looks like once it’s been solved:

**Start with Smallest Regions**. If a region is particularly small or has a unique shape, it might have limited options for star placements. Begin there.

**Mark Forbidden Cells**. Once you place a star, mark all adjacent cells as forbidden for star placement. This will help you visualise where stars can't go, which often reveals where they should go. You can use a dot, a small x, or shade these cells lightly to indicate they're off-limits for stars.

**Use Pencil Marks**. If you're unsure about a star's placement, mark it lightly or use a different symbol. If it doesn't lead to any contradictions, it might be a valid placement.

**Look for Forced Moves**. Sometimes, the configuration of a region or the placement of stars nearby will force a star into a specific cell. Look out for these as they can give quick progress.

Want to try your hand at Star Battle? We sometimes include this puzzle in our free Puzzle Weekly magazine – you should totally sign up for that if you haven’t already, as it puts 28 brand new puzzles in your inbox every week.

You can also find four levels of the puzzle in our* Jumbo Adult Puzzle Book* – which happens to include more than *500 puzzles* of 20 different varieties.

Tetromino is a logic puzzle that originated in Japan. The puzzle consists of a rectangular or square grid with symbols in some of its cells. The symbols can be triangles, diamonds, squares, or circles.

The objective of the puzzle is to divide the grid into regions, each comprising exactly four cells – known as tetrominoes – and each containing two symbols. And yes, tetrominoes are the same shapes you find in Tetris! For reference, here are the possible tetromino shapes:

These are just the basic shapes – tetrominoes can be flipped and / or mirrored, giving more possibilities.

Here’s what a simple Tetromino grid looks like:

- Each tetromino must contain exactly two different symbols.
- Tetrominoes of the same shape must all contain the same two symbols, though not necessarily in the same positions.
- Tetrominoes can be flipped, rotated or mirrored – they still count as the same shape whatever their orientation.
- When the puzzle is complete, every cell must be part of a tetromino, no cell can be left orphaned.

Here is the what the above example puzzle looks like once it’s been solved by drawing in the tetromino regions:

*Work from the edges.* Starting from the edges can be beneficial because there are fewer possibilities for tetromino placements. Once the edges are filled, it can make the inner sections easier to tackle.

*Start with known shapes.* Look for cells that already have symbols and try to form tetrominoes around them. Remember, each tetromino must have two different symbols.

*Rotation and mirroring.* Tetrominoes can be rotated or mirrored. This means that the same shape can appear in different orientations throughout the grid.

*Avoid isolating symbols.* As you form tetrominoes, ensure that you don't isolate any symbols. Every symbol should be part of a tetromino.

*Try using colour*. Some people find that colouring in the symbols can help them solve the puzzle more easily. That’s because some brains are wired to see colour differentials more prominently than shape differentials. Some people consider this cheating. We believe it’s a personal choice, so all of our Tetromino puzzles use unfilled symbols so that you have the option of colouring them.

*Check for consistency*. As you progress, regularly check to ensure that tetrominoes of the same shape have the same symbols. Adjust as necessary.

*Use deduction.* If you're stuck, try to deduce where symbols might go based on the remaining empty cells and the requirement for each tetromino to have two different symbols.

*Use elimination.* If you're unsure about a particular placement, consider all possible tetromino shapes that could fit in that space. By eliminating the ones that *don't* meet the symbol criteria, you can narrow down your options.

*Look for unique symbols.* If a particular symbol appears less frequently on the grid, focus on it. Since each tetromino must contain two different symbols, this can guide your placements.

*Avoid creating un-fillable spaces.* As you place tetrominoes, be cautious not to create spaces that can't be filled with a tetromino. If you notice such a space, backtrack and adjust your previous placements.

*Sketch out possibilities.* If you're solving on paper, lightly sketch out potential tetromino shapes before committing. This allows you to visualise placements without making permanent marks.

*Stay flexible.* Don't get too attached to a particular placement. If you find that a section isn't working out, be willing to erase or adjust your placements. Sometimes, a fresh perspective can help you see new possibilities.

*Break down larger grids.* For larger puzzles, it can be beneficial to break the grid down into smaller sections. Solve each section individually, then work to connect them together.

Like all puzzles, the more you practice, the better you'll become at spotting patterns and solving Tetromino puzzles more quickly. Practice with different difficulties. Start with simpler puzzles to get a feel for the game mechanics. As you become more confident, challenge yourself with more complex grids.

Want to try your hand at Tetromino? We have the perfect book for you! Tetromino: Volume 1 contains 100 puzzles over five levels of difficulty. We sometimes also feature this puzzle in our free Puzzle Weekly magazine – you should totally sign up for that if you haven’t already, as it puts 28 brand new puzzles in your inbox every week.

No Four in a Row is played on a square grid, which can vary in size according to the level of difficulty. Some of the cells in the grid are filled with either **X**s or **O**s. The objective of the puzzle is to fill in the rest of the grid with more Xs and Os such that there are never four (or more) of the same symbol appearing consecutively either horizontally, vertically, or diagonally.

Here is an example of a small grid:

Here is what the example puzzle looks like once it has been solved:

*Start with given symbols.* Consider the rows and columns the pre-filled symbols are in, and ensure that placing symbols around them won’t immediately violate the "no four in a row" rule.

*Look for safe plays.* Initially, try to place symbols in positions where it’s impossible to form four in a row due to the grid's boundaries.

*Blocking.* Sometimes, it’s beneficial to place a symbol simply to prevent a row of three (which would necessitate a block to prevent a row of four on the next turn).

*Mind the diagonals.* Diagonal lines can be tricky. Keep a close eye on the longer diagonals of the grid to make sure you’re not accidentally forming a line of four.

*Adjustments.* Be ready to reassess and retrace your steps if you find yourself in a position where you’re forced to place four symbols in a row.

*Consider future moves.* When placing a symbol, consider how it will impact future moves, especially in the surrounding rows, columns, and diagonals.

The more you play, the more you'll begin to notice patterns and strategies that work, making it easier to navigate through trickier puzzles.

Want to try your hand at No Four in a Row? We sometimes include them in our free Puzzle Weekly magazine – you should totally sign up for that if you haven’t already, as it puts 28 brand new puzzles in your inbox every week.

You can also find four levels of the puzzle in our *Jumbo Adult Puzzle Book* – which happens to include more than *500 puzzles* of 20 different varieties.

Number Cross uses a grid of numbers that at first glance might look a bit like a completed Sudoku puzzle. But contrary to Sudoku, Number Cross is a mathematical puzzle.

The goal is to cross out numbers *inside* the grid so that the remaining numbers in each row and column add up to the numbers *outside* it. Here's a a small example Number Cross puzzle:

Here’s what that puzzle looks like once it’s been solved:

*Start with unique numbers*. If a row or column total can only be made by a unique combination of numbers present in the grid, start there.

*Look for the smallest and largest totals*. Small totals mean more potential numbers that can be immediately crossed out. For example, if the total for a row is ‘2’, then anything larger than ‘2’ can be crossed out in that row. Similarly, very large totals usually require keeping the larger numbers in the grid, thus narrowing down your choices.

*Track remaining options*. In harder puzzles, for rows or columns where you're unsure of which numbers to cross out, it can help to make a list of possible combinations that add up to the required total. As other parts of the grid get filled in, some of these options will become invalid, leaving you with the answer.

As with all logic puzzles, practicing improves performance. The more puzzles you do, the better you well become at spotting common patterns and at recognising possible combinations.

Want to try your hand at Number Cross? We sometimes include them in our free Puzzle Weekly magazine – you should totally sign up for that if you haven’t already, as it puts 28 brand new puzzles in your inbox every week.

You can also find four levels of Number Cross in our *Jumbo Adult Puzzle Book* – which happens to include more than *500 puzzles* of 20 different varieties.

Shirokuro is played on a square grid that contains black and white circles. Here’s what a small Shirokuro puzzle looks like:

The goal of Shirokuro is to connect all the black and white circles into pairs by drawing a line between them either horizontally or vertically, according to some rules:

- Each circle must be connected to one other circle of the opposite colour.
- Circles must be connected by horizontal or vertical lines, but never diagonal ones.
- Connecting lines must not cross over each other or pass through other circles.

This is what the example puzzle looks like once it’s been solved:

*Start with the corners*. If a circle is near the corner, or even an edge, it may have limited directions it can connect in.

*Limit options*. If connecting a circle in one particular direction would make it impossible to connect another circle, then reconsider that choice.

*Work incrementally*. Don't try to map out long connections immediately. Work step by step, ensuring that each connection you make doesn't block future connections.

*Avoid loops and crossings*. Lines cannot cross over each other. If you see a setup that's leading to this scenario, you'll need to adjust.

As always, the more Shirokuro puzzles you solve, the better you'll get at spotting patterns and strategies.

Want to try out some Shirokuro? We have options. We sometimes include them in our free Puzzle Weekly magazine – you should totally sign up for that if you haven’t already, as it puts 28 brand new puzzles in your inbox every week.

You can also find four levels of Shirokuro puzzles in our *Jumbo Adult Puzzle Book* – which happens to include more than *500 puzzles* of 20 different varieties.

Calcudoku is a mathematical and logic puzzle similar to Sudoku. It’s played on various grid sizes, usually from 4x4 to 9x9, though it can go even larger.

The size of the grid dictates the numbers you’ll use to fill out the puzzle. For instance, in a 4x4 Calcudoku, you'll use the numbers 1 to 4, and in a 6x6, you'll use the numbers 1 to 6.

Here’s an example of a small Calcudoku puzzle:

The objective of Calcudoku is to fill the grid with numbers so that:

- Each row and each column contains only unique numbers, without repetition (the same as Sudoku). For example, in a 4x4 Calcudoku, each row and each column should contain the numbers 1, 2, 3, and 4 with no repetitions.
- Each outlined block must satisfy the mathematical operation given by its clue. For example, if a block of two cells has the clue ‘3+’, then the numbers in those cells must add up to 3. If a block of three cells has the clue ‘6x’, the numbers in those cells, when multiplied together, should give a product of 6.

Here’s what the earlier example puzzle looks like when completed:

Because Calcudoku shares similar rules to Sudoku, we highly recommend becoming familiar with solving Sudoku before moving on to this puzzle. As you begin to solve cells in a Calcudoku grid, you can use many regular Sudoku techniques to help you solve the rest of the puzzle. Indeed, in anything beyond the most basic puzzles, you’ll *need* to use Sudoku techniques. You can find our complete three-part Sudoku tutorial here.

The Calcudoku grid is divided into several outlined blocks, and each block contains a mathematical clue in the top-left corner. This clue might be a number on it’s own (ie the contents of the cell) or it could be a number followed by an operation sign (e.g., ‘12x’ or ‘3-’), in which case the calculation must be performed on the numbers that are entered in the block.

The puzzles we publish use a variety of mathematical operators. Our kids puzzles usually only include additions, but as the difficulty level increases, so do the possible operators.

- Addition (+): The numbers in the cells of the block must sum up to the given number.
- Subtraction (-): For blocks with only two cells, one number subtracted from the other must equal the given number. Note that order matters, so if the clue is ‘3-’, and you have a 5 and a 2, then 5 - 2 = 3 is correct, but 2 - 5 is not.
- Multiplication (x): The numbers in the cells of the block must multiply to give the given number.
- Division (÷): Like subtraction, this usually appears in blocks with two cells. One number divided by the other should equal the given number. Again, order matters.

A well-designed Calcudoku puzzle (such as those we publish) has only one unique solution, and it can be reached through logical deduction. There is never any need to guess.

*Start with the obvious*. If you see a block in a 5x5 puzzle with the clue ‘5x’ and it contains only two cells, then those cells must be filled with 5 and 1 (in some order), because that's the only way two distinct numbers between 1 to 5 can multiply to give 5.

*Use a process of elimination.* If you've determined certain numbers for some cells, use that information to deduce the numbers for neighbouring cells, especially within the same row or column.

*Consider block position.* For instance, in a 6x6 Calcudoku, a block with the clue ‘1-’ must contain a 2 and a 1 (because 2 - 1 = 1). If that block spans two rows or columns, and one of them already has a 2, then the 2 in the block must go in the other row or column.

*Use Sudoku strategies*. Because the two puzzles share common rules, you can use all valid Sudoku strategies to help solve Calcudoku. The more cells you fill in, the more these strategies will be helpful.

*Practice*. As with all logic puzzles, the more you practice, the more patterns and strategies you'll recognise, making it easier to solve more challenging puzzles.

Want to try out some Calcudoku? We have you covered. We sometimes include them in our free Puzzle Weekly magazine – you should totally sign up for that if you haven’t already, as it puts 28 brand new puzzles in your inbox every week.

You can also find four levels of Calcudoku puzzles in our *Jumbo Adult Puzzle Book* – which happens to include more than *500 puzzles* of 20 different varieties.