Why Do Leaves Float on Water?

From the firm path beside the little woodland pond, Puschel saw a dry leaf that had fallen onto the water by itself. It stayed on top and slowly turned in a circle. The two kept back from the edge.

Puschel“It has no boat and still floats. Perhaps it can paddle in secret?”

Flauschi lifted the green explorer’s magnifying glass and looked towards the broad, light leaf. From their safe spot, they could not see whether air was trapped beneath it.

Flauschi“Water pushes things upwards. This push is called buoyancy. At the top, water also holds together a little like a thin, stretched skin. This is called surface tension. It can help hold up a very light leaf, but not you. Sometimes air beneath a leaf helps too.”

Puschel noticed a small round stone resting on the clear bottom nearby. He pointed without leaning over the water or throwing anything into it.

Puschel“The stone took the shortcut down!”

Flauschi explained that an ordinary stone is heavier than an equal-sized amount of water. We say it is denser. The water’s upward push is not enough to hold up the stone, so it sinks.

Flauschi“Being light or heavy is not the whole story. A ship has lots of space with air inside. When it sits in water, it pushes water aside. This can give it enough upward push to float.”

After a while, one edge of the leaf dipped lower and water washed over it. Flauschi explained that some leaves absorb water and sink later, while others float for longer.

Puschel“So it was a leaf boat for a while. I shall let it travel on without disturbing it.”