1Two things to know first
Rock is hard, but water never stops
You need two small ideas. Watch each one happen:
Mountains are made of rock
Every mountain and cliff is solid stone — chunks of it, big and jagged, broken off over time.
Rivers bash rocks together
Rushing water rolls rocks along the riverbed, knocking them into each other again and again and again.
2Look closely at the grains
A fresh chip vs. a well-traveled grain
Just broken off 🪨
A chip that just snapped off a cliff is sharp and jagged, with pointy corners. It hasn't gone anywhere yet.
Traveled a long way ⏳
A grain that tumbled down a river for ages is smooth and round — its corners are worn off. Keep this in mind…
3Watch a rock get smoothed
More tumbling, rounder rock
Here's one jagged rock in a tumbling river. Slide to add more tumbling time and watch its sharp corners get knocked off, bit by bit.
Each knock chips off a corner. The longer it tumbles, the rounder — and smaller — it gets.
4Now predict the beach
Where did a whole beach of sand come from? 🏖️
You've seen one rock get rounder. But a beach is millions of tiny grains. Make your guess, then send a boulder down the river and watch what it turns into.
Guess before you watch
Grab a handful of beach sand — millions of tiny grains. Where did nearly all of it come from?