Is Stack the Jelly Friends free?

Yes. It runs in your browser on phones, tablets and computers, with no download, no account and no ads in the game.

How does the soft-body physics work?

Each jelly is a ring of point masses pulled towards its best-fitting shape by Hooke’s-law springs (shape matching), with its area held constant like a gas at fixed temperature so it bulges when squashed. Gravity, damping, friction and wind drag act on every point.

Why does jelly jiggle?

Gelatin is very soft in shear, with a shear modulus of about 1 to 10 kPa. Waves cross it slowly, at about a metre or two per second, so a dessert-sized jelly wobbles a few times a second before damping calms it.

What makes a jelly fall off the plate?

Rolling off an uneven jelly below, a squashed and leaning base, wind gusts, or the rocking cake stand. Firm jellies at the bottom and centred drops help.

Is my progress saved?

Yes, your unlocked levels, stars, stickers and best tower are kept in your browser on this device. Nothing is sent anywhere.

Stack the Jelly Friends

Squishy friends, wobbly tower. Don’t let anyone slip! Back to the game

How to play

A sleepy paper cloud carries one jelly friend at a time. Move the cloud left and right (hover with a mouse, or drag with a finger), then let go to drop the jelly onto the cake stand. Stack the friends into a tower that reaches the dashed goal line, and hold it there, still, for a second and a half. If anyone slips off the plate, the level is over (they’re fine, just a bit embarrassed). Each level gives you a fixed number of jellies; finish with two to spare for three stars. You can always see who is coming next in the top corner.

Settled jellies love attention: poke one and it giggles and jiggles, or press and drag to give it a gentle shove. Careful, a giggle at the bottom shakes everyone above. Later levels add wind gusts and a cake stand that slowly rocks from side to side.

Keyboard: Left and Right arrows (or A and D) move the cloud, Space, Enter or Down drops, P pokes the newest jelly, and R restarts the level.

The physics of a jiggly jelly

Every jelly is simulated as a soft body: a ring of 14 to 20 point masses, each pushed by gravity (9.81 m/s²), wind and its neighbours. Several hundred times a second the game works out the jelly’s best-fitting position, turn and squash (a method called shape matching), and every point is pulled part of the way back towards where that shape says it belongs. That pull is a spring obeying Hooke’s law, F = −k·x: the further a point is pushed out of place, the harder it is pulled back. The squash itself also springs back towards round, which is the jiggle.

To stop a jelly from squashing flat, the model keeps its area constant, like gas trapped at a fixed temperature: by the ideal gas law, PV = nRT, so pressure times volume stays the same, and a jelly squashed to 90 percent of its size would push back with 11 percent more pressure. Real jelly is mostly water, which hardly compresses at all, so real jellies change shape but keep almost exactly the same volume. That is why a squashed jelly bulges out at the sides.

Why gelatin jiggles

Gelatin is a mesh of collagen proteins holding a lot of water. Its stiffness against shearing, the shear modulus G, is only about 1 to 10 kilopascals for a dessert jelly, roughly ten million times softer than glass. Shear waves travel through it at √(G/ρ): with a density of about 1,050 kg/m³ that is only 1 to 3 metres per second. A wobble is a wave bouncing back and forth across the jelly, so a jelly the size of a strawberry friend jiggles three or four times a second. In the game the soft strawberry jellies wobble at about 3.5 Hz (G ≈ 3.5 kPa) and the firm limes at about 5.5 Hz (G ≈ 7 kPa). Their outlines are kept a little firmer than a real dessert so a tower can stand, but the wobble you see runs at those real jiggle rates.

A jiggle fades because of damping: friction inside the gel turns the motion into a tiny amount of heat. The strawberry jellies are lightly damped, so they keep wobbling the longest. Friction between jellies and against the plate follows Coulomb’s rule, and on a tilted plate a jelly stays put while tan θ < μ.

Wind pushes with the drag force F = ½ρC_dAv², which grows with a jelly’s cross-section, while its weight grows with its volume. So the force per kilogram is larger for small things: a 13 cm blueberry feels nearly twice the push per kilo of a 24 cm strawberry.

Tips

Put firm lime jellies at the bottom. A soft strawberry under a heavy load squashes, leans and tips the tower.
Drop straight down onto the middle of the jelly below; even a little offset makes it roll.
Wait for the wobble to settle before the next drop. A calm tower is a tall tower.
On windy levels, aim slightly upwind and save the tiny blueberries for the very top, or for calm moments.

Love wobbly things? Try stacking crates in ../stack-it/, or stretch a soap film in ../blow-a-bubble/.

Questions people ask

Is Stack the Jelly Friends free?: Yes. It runs in your browser on phones, tablets and computers, with no download, no account and no ads in the game.
How does the soft-body physics work?: Each jelly is a ring of point masses pulled towards its best-fitting shape by Hooke’s-law springs (shape matching), with its area held constant like a gas at fixed temperature so it bulges when squashed. Gravity, damping, friction and wind drag act on every point.
Why does jelly jiggle?: Gelatin is very soft in shear, with a shear modulus of about 1 to 10 kPa. Waves cross it slowly, at about a metre or two per second, so a dessert-sized jelly wobbles a few times a second before damping calms it.
What makes a jelly fall off the plate?: Rolling off an uneven jelly below, a squashed and leaning base, wind gusts, or the rocking cake stand. Firm jellies at the bottom and centred drops help.
Is my progress saved?: Yes, your unlocked levels, stars, stickers and best tower are kept in your browser on this device. Nothing is sent anywhere.