Gravity snake puzzles, explained by a hungry worm

• 1Square per move. The worm crawls, it doesn't jump.
• 7Worlds, from the vegetable patch to the workshop.
• 0Timers. Take your time getting it wrong.

The rule: eat, then go home

The worm moves one square at a time: up, down, left or right. The head chooses, and the body follows exactly in its tracks. It has to eat every apple in the level, then go into the burrow. As long as a single apple is left, the burrow stays shut. Crawling past it achieves nothing: you'll have to come back.

Each apple adds one segment to the worm, straight away: on the move where it takes a bite, its tail stays where it was, and the worm is one square longer at once. It sounds like a trivial detail. Yet it makes or breaks quite a few tight passages: eat an apple just before the way in, and the worm no longer fits.

The worm never turns around: its head can't go into its own body. So a dead end is a death trap, best spotted before you poke your nose in. The good news is that an impossible move costs nothing. Go ahead and try: the counter won't budge.

Gravity, your worst enemy and your best friend

The board is seen from the side, and anything with nothing underneath it falls. The worm stays up as long as just one of its segments is touching something solid: the ground, a crate, an apple it hasn't eaten yet, the burrow. So with support under its tail, it can reach out over the drop without falling, like a living diving board.

When it stops touching anything, it drops in one piece, keeping its shape. It feels like a punishment. It is often the solution: a well-placed fall slips the worm into a gap it could never have reached by crawling.

Above all, its own body is the ladder. The head can rear up one square above the rest, and the worm can climb over itself to go up two storeys or step across a hole. The longer it is, the higher it climbs. The longer it is, the more trouble it has squeezing round corners, too. The worm is a compromise.

One last surprise: the board is a window, not a box. The worm can leave through an edge and come back, and it only dies if it goes entirely below the board. Some levels have no other way through. Yes, that's deliberate. No, it isn't cheating.

What stings, what shifts, what supports

• Spikes kill, whether you crawl onto them or land on them. The fall you thought was safe is their speciality.
• Apples hold the worm up until it eats them. Biting into the one you're standing on means pulling the ground out from under your own feet. Well, your own belly.
• Crates move one square per push, and they fall when nothing holds them up. Pushed off a ledge, a crate becomes a step. Pushed onto spikes, it covers them and makes a bridge. Pushed off the board, it is lost, and the level goes with it.
• A crate against a wall won't move any more: it's a fixed step, on the house.

Crates can never be pulled. Every push is final, and a crate stuck on the wrong side can make the level impossible without warning you. That's what the Restart button is for, and it's free.

The traps everyone falls into

Once you understand the rules, the difficulty no longer comes from the unknown but from the way they combine. The levels are built so that the first idea that comes to mind is wrong. The all-time classics:

• the order of the apples: eating the nearest one first makes the worm longer at the wrong moment, and the next passage won't let it through any more;
• using your own body as a prop: it shifts on your next move, so it doesn't exist;
• the tail that moves on: you're counting on it to hold you up, and off it goes with the rest;
• the load-bearing apple: the floor disappears the moment you bite;
• the one-way fall: you drop down a storey you can't climb back up;
• the blind trap: the mistake only shows three or four moves later, when it is too late to admit it.

The method

1. Look before you touch. Find the burrow, the last apple and the path that links the two. The end of the level often tells you where to begin.
2. Count your length. Each apple adds a segment. Before you bite, ask yourself whether the worm will still fit where it has to go next.
3. Save the highest apple for when you're all grown up. A long worm climbs higher. The apple in the attic is often the last one you eat.
4. Look for what holds you up, not what's in front of you. Before each move, check which segment will still be touching something. If none will, you fall, and that may be exactly what you needed.
5. Only push a crate if you know where it will land. A pushed crate never comes back. Ask yourself what it will be good for once it gets there.

Why every level has a solution, and an honest par

Every Worm level goes through a solver before it's published. The program explores every possible position of the worm, the crates and the apples, and checks two things: that the level has a solution, and that its par is the true minimum, the smallest number of moves that gets you to the burrow. So par isn't a generous estimate from someone who got lucky. It's the best anyone can possibly do.

The solver also measures the traps. A good level isn't a long level, it's a level where the first path that comes to mind fails. You can make a sixty-move level that is mostly walking. We did, we got bored, and we cut out the walking.

Where this kind of game comes from

The snake that keeps getting longer is an old idea. In Blockade, back in 1976, there wasn't even anything to eat: you grew all by yourself. Food arrived with the clones that followed, one of which was already called Worm (we didn't invent a thing, not even the name), and then, from 1997, the snake made its way onto Nokia phones. But that snake lived on a grid seen from above, with no up or down, and its only enemy was its own tail.

Add gravity and everything changes. The best-known game of the genre is probably Snakebird, by the Swedish studio Noumenon Games, released on PC on 4 May 2015 and then on mobile in 2016: snakebirds that grow longer by eating fruit, fall the moment they stop touching anything, and have to reach an exit. It has a reputation for being brutal, and it deserves it.

Since then, the recipe has been picked up by plenty of mobile and web games, often with a worm and apples, several of them under the name Apple Worm. Worm has no connection with any of them. It keeps the basic rule and does things its own way: levels checked by a solver, a par that is the true minimum, and a garden where you can build your own.

In this game

The gesture

Swipe in the direction you want, or use the on-screen D-pad, or the arrow keys. One move per gesture, never more: a long swipe only moves you one square. That way, an overexcited thumb can't send the worm three squares at once, straight into the spikes.

Restart, undo, ask for a hint

Restarting is free, always. Undoing a move costs a short video, one per undo: an ad in the app, and on the website a little carousel of our other games that you can skip after ten seconds. The hint works the same way: it shows the next three moves of an optimal solution from where you are, or tells you straight out that you've hit a dead end.

Stars

Three stars if you match par, two for up to a quarter more moves (always with at least three moves' leeway), one beyond that. Beating par is impossible: the solver saw to that. The leaderboard adds up your stars.

Seven worlds

The vegetable patch to learn the ropes, then the thorns, the traps, the crates, the galleries, the far garden and the workshop. A world opens once you've won a little over half the levels in the previous one: nobody gets stuck behind a single fiendish level.

Your garden

When you've finished every level, an eighth world opens: your garden. There you build your own levels with the game's editor, you play them, and if you're proud of one, you can suggest it. We take a look, and if we like it, it joins the game under your name. Nothing gets published until a human has played it.

Frequently asked questions