Snake

Snake (sometimes also called Tunnel) is a spatial awareness path-finding logic puzzle. The aim is to connect the snake’s head with its tail by filling in grid cells following certain rules. Here’s what a very easy Snake puzzle looks like:

Rules of Snake

The rules of Snake are easy to remember:

The snake’s body can be filled in horizontally and vertically
The body must never touch itself, not even diagonally
The numbers outside the playing grid tell you how many cells must be filled in for a row or column

In easier level puzzles, every row and column is labelled. As the difficulty ramps up, some rows and columns are blank, making it harder to determine the snake’s path.

Here’s what our earlier example puzzle looks like once solved:

The solution to the small example puzzle

Solving Snake Puzzles

Solving these puzzles requires a combination of elimination and forced placement based on logical deduction. There is no single ‘correct’ way to solve a Snake puzzle, rather, you need to have a variety of techniques available to you.

Although the goal is to connect the snake’s head to its tail, there is no particular need to start at either the head or the tail; sometimes it makes more sense to start somewhere in the middle, solving different sections at a time until eventually they all link up.

Here are some tips to help you tackle Snake puzzles, then we’ll look at a worked example below.

As you find cells that you know cannot contain part of the snake, it is useful to mark them off. As you narrow your options in this way, you will find places where the snake must go.
Check for extremes. If a row or column is marked as 0, you can eliminate all the cells within it. Similarly, a row or column labelled with a number that matches the dimension of the grid (eg: labelled 6 in a 6x6 grid) can be fully filled in. However, both these scenarios are very rare and unlikely to appear in anything but the simplest puzzles.
Look for rows and columns labelled as 1. It is useful to highlight these because they effectively split the grid into unique sections. You can think of these rows and columns as one-way crossings – once the snake crosses one, it cannot return. Thus it becomes necessary to complete each side of a ‘1’ column or row before crossing it.
Bear in mind that crossing a ‘1’ column or row requires at least three consecutive cells to be filled in. This further narrows your possible crossing places.
Rows or columns labelled 2 are also constrained. You cannot perform a ‘U-turn’ within them, because that would require at least 3 cells due to the fact the snake cannot touch its own body. This can reduce potential crossing places.
If you locate three or more filled cells in a row, you can block out the middle cells to either side. Again, the rule about the snake not touching its own body means the snake cannot return to those cells.
Remember that as soon as the snake body touches the head or tail, it cannot go any further. So be careful not to run into either too soon.
Cross out clue numbers outside the grid as you complete rows and columns; it helps to remind you where you can no longer go.

Worked Example

We will work through a sample puzzle from beginning to end. As with all examples, remember that there’s no single path to the solution. This is just one way to get to the end.

This is the Snake puzzle we are going to solve. At 8x8, and with every row and column labelled, it’s quite easy.

There are no extreme values on this 8x8 grid (no rows or columns labelled with a 0 or an 8).

We do however, have a ‘1’ row and a ‘1’ column. Because these are one-way crossings, it’s useful to highlight them. Once the snake’s body crosses each of these, there’s no turning back.

This allows us to immediately cross off three cells. The two to the right of the tail cannot be used, because it takes three cells to cross a ‘1’ column, and that would violate the 2 for that row. We can also eliminate the cell to the left of the head, for the same reason. That means the body extends from the head either up or down. Either way will allow us to fulfil the 2 clue for the column containing the head. Which way to go?

We have to go down. Why? Because the row above the head requires three cells to be filled in. Crossing the ‘1’ column would use our full complement. We would have to get back down to the bottom row to fill in the six cells that must be filled there, and that would violate the ‘2’ row, and the ‘3’ row on our way back. The only way to fill in the ‘6’ row is to go there directly from the head.

Once on the bottom row, we know we fill three cells to cross the one-way ‘1’ column. In fact, we can also deduce that we must place all six required cells in that row because there’s no valid way to leave the row and return. In other words, we couldn’t go across three cells, up a couple, across again, and back down, because that would violate the rows above.

This one move has completed a row and two columns for us. Where do we move next?

We know we have to move up the ‘7’ column, because we’ve filled the ‘6’ row. Going up one cell completes the ‘2’ row (with the head), so we must go up another cell – there’s no choice.

Now we do have a choice though. Because we have a one-way barrier ahead of us (the highlighted ‘1’ row), we know we must complete the ‘3’ row before moving upwards. So do we go left or right?

The one-way barrier means that once we turn upwards, we will have to continue for at least another two cells. Going right would violate the rules in two ways. Firstly, we would be filling three cells in a ‘2’ column. Secondly, we would end up touching the snake’s tail (diagonally). As the snake cannot touch itself, this would be an invalid move.

So we have to go left…

…then we have to cross our one-way barrier row by going up three cells, completing another column.

We can’t go up any more, so we have to go right…

…and then upwards, because we’ve completed the three cells allowed in this row, and also because we have to complete our ‘7’ column. We’ve knocked off quite a few rows and columns now.

It’s a clear run to the end. We have to fill four cells in the top row, and there’s only one way to do that…

…which means we can drop down to the tail, completing the puzzle. Easy! Ready to have a go yourself? Read on!

Try Snake Yourself

We’ve put together a taster of four puzzles for you to try out, including the example above. You can download and print the PDF below. Solutions are included, but no cheating!

Download Our Snake Taster

Where To Play

Puzzle Weekly Presents: Snake - Front Cover

Finished the taster and want more Snake in your life? No problem! Get 120 carefully crafted puzzles set over seven levels in Puzzle Weekly Presents: Snake – it's great value!

We publish Snake puzzles in Puzzle Weekly from time to time. Puzzle Weekly is our free weekly puzzle magazine – find out more, and get your copy, here