1What a pile of sand is really doing
Sand can only get so steep — and a pile climbs right up to that line
You only need two ideas. Watch each one:
There's a "too-steep" line
Sand can only pile up so steeply. Tip it past a certain steepness and grains let go and slide. Every kind of sand has its own tipping steepness.
The pile climbs up to it
Drip grain after grain and the slope creeps higher and higher, all on its own, until it's sitting right at that tipping steepness — balanced on the edge. Nobody put it there. The dripping did.
2Two piles, two rules
The polite pile vs the poised pile
Imagine two piles that follow different rules for what a grain does when it lands:
The polite pile
Every grain just nudges its neighbours a little, so the pile spreads out flat and stays gentle. It never gets steep — so it never tips.
The poised pile
Grains stack up until the slope hits the tipping steepness, then it balances right on that edge. Now one grain can start a slide.
3Your turn — drip the sand
Drip grains and watch the slope climb to the edge
Tap to drop a grain, or hold down drip sand to pour. Watch the steepness meter climb — and notice the pile starts shedding little slides all by itself once it reaches the edge.
4Now the real test
Two piles are poised at the edge. Drop one more grain. 🏔️
Both piles are built right up to the edge. We'll drop the same plain grains on each — the polite-rule pile on the left, the poised-rule pile on the right — and measure how big each slide is. Guess first, then run it.
Guess before you drop it
A sandpile has built up to its steepest possible slope. You drop one more ordinary grain. What happens?