Stack the Sky
Drop shipping containers from a swinging crane and build the tallest tower you can. It bends under its own weight, leans toward every container you put down badly, and eventually goes over.
About This Game
One button. A crane swings a container back and forth above your tower, and you press once to release it. That is the entire control scheme, and it takes about ninety seconds to understand and considerably longer to stop being bad at.
The reason it is hard is that the container does not fall straight down. It keeps whatever sideways speed the cable gave it, so releasing it directly over your target sends it well past your target. You have to let go early and let it drift the rest of the way — a lead calculation your hands work out long before your head does.
And the tower is not a stack of decorations. It bends like a real cantilever, leaning further at the top than the bottom, and it remembers every container you placed slightly off. A run does not end because you missed. It ends because thirty drops ago you were two centimetres out, and the tower has been quietly getting worse about it ever since.
How It Decides To Fall
Nothing here is a random failure roll. The game runs the same check a structural engineer would, at every joint in the tower, on every frame:
- Take any joint — say between container 12 and container 13.
- Work out the combined centre of mass of everything above it, including how far the sway has pushed each container sideways at that moment.
- If that centre of mass is still over the container underneath, the joint holds. If it passes the edge, the joint hinges and the tower comes down from there.
This is why a tower can stand for twenty floors and then fail without being touched: the sway swings the mass a little further each cycle, and eventually the worst joint runs out of margin. The stability meter shows you exactly how much of that margin is left at the weakest point.
It is also why the collapse always starts somewhere specific rather than everywhere at once. Watch where it hinges — that is the container you misplaced.
The Sway
Real tall structures do not wobble like a rigid pole on a spring. They bend, curving more toward the top, and they get slower as they get taller — a taller building has a longer natural period, which is why skyscrapers sway lazily rather than buzzing.
The tower here does the same thing. Displacement follows a cantilever's first bending mode exactly — a quarter of the way up, a container moves about a sixteenth as far as the top does — so the top swings wide while the base barely moves.
The rate is deliberately not exact. A real uniform cantilever slows down with the square of its height, which would put a sixty-container tower on a sway period of over a minute: correct, and completely unplayable. It is softened here to run from roughly one and a half seconds at the bottom to about five at the top. The direction of the effect is real, the steepness is tuned. Early on the sway is quick and easy to time around; by floor forty it is a slow lean that takes several seconds to come back, and you will find yourself waiting for it rather than dropping into it.
| What feeds the sway | Effect |
|---|---|
| Landing impact | A hard, off-centre landing kicks the tower sideways |
| Wind | Pushes constantly and gets stronger with height |
| Permanent lean | Badly placed mass shifts the rest position, not just the wobble |
| A clean drop | Damps it. Perfect placements actually calm the tower down |
How To Play
- Tap the screen, click, or press Space to release the container. That is the only control.
- Release before the swing reaches the middle. The faster it is swinging, the earlier you go.
- The faint marker on the tower shows the predicted landing spot. It fades out after the first ten containers — use those ten to calibrate.
- Land within a few centimetres of centre for a perfect: it scores more, builds a combo, and damps the sway.
- Watch the stability meter, not the tower. By the time a lean looks bad it usually is.
Tips For A Tall Run
- Correct a lean by deliberately dropping to the opposite side. Two off-centre containers on opposite sides are more stable than one off-centre container.
- Do not chase the swing when it is at its widest. Wait a beat; the release window is the same size but your timing error costs less near the extremes.
- Perfect drops are worth more than the points suggest, because damping the sway is what buys you the next ten floors.
- In strong wind, aim slightly upwind. The container is exposed for the whole fall, not just the moment of release.
- When the meter goes amber, stop pushing for height and spend two containers straightening up.
Why This Exists
Everyone who has stacked blocks as a child has an intuition for towers, and it is mostly wrong in one specific way: we think about the top of the tower. We line up the newest block with the one below it and feel like we have done our job.
Towers do not fail at the top. They fail at whichever joint has the least margin, which is usually somewhere in the lower third, decided by a mistake you made ages ago and forgot about. This is a small, fast, extremely repeatable way to have that pointed out to you.
Frequently Asked Questions
What is Stack the Sky?
Stack the Sky is a free one-button browser game. A crane swings a shipping container above a growing tower and you press once to release it. The tower bends under its own weight, leans toward wherever you put the mass, and eventually falls over. There is no finish line, only how high you got.
Why does my tower fall over when nothing hit it?
Because it was already unstable and the sway finally caught up with it. The game checks every joint in the tower and asks whether the combined centre of mass of everything above that joint still sits over the container holding it up. Each slightly-off container shifts that centre, and one small drop eventually pushes a joint past its limit.
When is the best moment to release?
Earlier than feels right. The container keeps the sideways speed it had on the cable, so releasing it directly over the target sends it past the target. You have to release before the swing reaches the middle and let it drift the rest of the way, which is a lead calculation your hands learn faster than your head does.
What is the stability meter?
It shows how close the worst joint in your tower is to tipping, as a percentage of the margin it has left. Green means comfortable, amber means one bad drop from trouble, and red means the tower is being held up by the sway happening to be leaning the right way at that instant.
Does the wind actually matter?
Yes, and it gets stronger the higher you build. It pushes the container sideways during the whole fall, so a drop that would have been perfect at ground level lands short or long near the top. It also pumps the tower's sway, which is what usually finishes off a tall run.
Does it work on mobile?
Yes. It works on modern mobile browsers including Chrome, Safari and Firefox, and it is built for one thumb. Tap anywhere on the screen to release a container. Portrait is fine here, since the tower grows upward rather than sideways.
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