Why can one tiny grain start a whole avalanche?

Drip sand onto a pile, one grain at a time. For ages, each grain does almost nothing. Then one ordinary grain — no bigger than the rest — sets off a slide down the whole side. So which grain was the cause? Let's drip some sand and find out.

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:

Gentle rule

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.

Real-sand rule

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.

How steep the pile isjust starting
Keep dripping — see how high the slope climbs.

4Now the real test

Can one gentler step stop a chain reaction?

Try a new, exact sand model. Each pile has 30 columns. A column sends one grain to the next whenever it stands MORE THAN two grains above its neighbor. All the height differences are two, except the left pile’s third step, whose difference is only one. Drop one grain on top of each.

Guess before you drop it

In the left pile, will the slide pass the gentler third step and keep going, or stop there? Remember: a difference of two is still stable; it must be MORE THAN two to move.