Why are some beaches sandy and some pebbly?

Picture a beach where soft sand squishes between your toes. Now picture another beach, where smooth round stones clatter under your feet. Both beaches sit by the same big sea.

So why does one beach have soft sand, and another have smooth pebbles? Let's find out together.

Sand and pebbles are both bits of rock. A pebble is a big bit that fits in your hand. Sand is a pile of grains, which are tiny bits of rock smaller than a crumb.

A pebble feels heavy in your hand. One sand grain is so light, you can barely feel it on your fingertip. Heavy things and light things move in very different ways.

Waves come in different sizes. Out on the wide, open sea, the wind pushes up big, strong waves. Some beaches sit where the land curls around the water like a hug. The land keeps the big waves out, so the waves there stay small and gentle.

Strong waves push hard. When each big wave slides back into the sea, it pulls the light sand grains away with it. Which bits do you think stay behind on the beach?

The heavy pebbles stay! The waves cannot carry them off, so the pebbles just roll and rattle. Day after day, the waves take the sand away, and only pebbles remain.

Now think of a beach with gentle waves. Small waves push the sand grains softly up onto the beach. Their pull back is too weak to carry the grains away, so the grains pile up into a soft, sandy beach.

Strong waves pull the light sand grains away, so only the heavy pebbles stay. Gentle waves are too weak to pull the grains back, so soft sand piles up.

On the pebbly beach, the smooth stones rattle as each big wave slides back. Far away, in a calm curl of land, little waves lap softly over the sandy grains.