Where Did the Puddle Go?

Part One · The chalk line

After the rain, there was a big puddle on the sidewalk in front of Calla's house. It was so big that her boots splashed all the way to the middle. Calla got a fat piece of chalk and drew a line around the edge of the water. "Now I will know," she said, "if this puddle goes anywhere."

At lunch, Calla ran out to look. The puddle was smaller! There was a ring of dry sidewalk between the water and her chalk line. Nobody had swept it away. No dog had come by for a drink. "Mom!" she yelled. "Somebody is taking my puddle!" Mom came out and looked up at the sky. "Somebody is," she said. "The sun."

Part Two · Up in the air

The sun was warming the puddle. Bit by bit, the water was turning into a gas called water vapor. The pieces of vapor are too tiny to see, and they float up into the air. Water rising away like this has a big name: evaporation. So the water had not gone away. It had gone up, and some of it was floating all around Calla.

Calla waved her hand through the air. It felt like nothing at all. "There is no water here," she said. So Mom filled a glass with ice and water and set it on the warm front step. Soon the outside of the glass was covered in little beads of water. Calla touched one with her finger. "Where did that come from?"

"It came out of the air," said Mom. The cold glass made the vapor near it cold, too, and cold vapor turns back into drops of water. This is called condensation. Calla put her mouth close to the glass and went huff. A foggy spot grew on it. Her own breath had water vapor in it!

Part Three · Clouds and rain

In the afternoon, Calla lay on the grass and watched a big cloud. "Is that cloud made of puddles?" she asked. In a way, it was. Warm air carries water vapor up high, where the air is cold. Up there, the vapor turns into tiny drops, and each drop starts on a speck of dust or salt. Lots of them together make a cloud.

Cloud drops are so small and light that rising air holds them up. They bump into each other and join, and they grow bigger and bigger. It takes millions of cloud drops to make just one raindrop. Calla tried to count to a million in her head. She got to forty-two and then she gave up.

In drawings, a raindrop often has a point on top. Real raindrops do not look like that. Small ones are round, like a tiny ball. As a bigger drop falls, the air pushes its bottom flat, so it looks like a hamburger bun. The biggest drops puff out like a little parachute, and then they split into smaller drops.

Part Four · Round again

The next morning, it rained again, and Calla stood under her umbrella to watch where the rain went. Some of it ran down the street and into a drain. Some of it soaked into the garden dirt and was gone. "It moves under the ground," Mom said, "slowly, between bits of soil and rock, like water in a sponge."

Calla's tomato plant drank water from the wet dirt with its roots. But where did the water go next? Mom tied a clear plastic bag around a few of its leaves. An hour later, the inside of the bag was misty and wet. The plant was letting water vapor out of its leaves, back into the air.

Calla drew it on the sidewalk. The sun lifts water into the air. Up high, the water makes clouds. The clouds drop rain, and the rain runs off, or soaks in, or gets drunk by plants. Then the sun lifts it up again. "It never stops!" said Calla. This is called the water cycle, and it has no start and no end.

Almost all of Earth's water is in the ocean. But the same water goes up and comes down again and again, all over the world. The rain in Calla's puddle may once have fallen on a farm far away, a hundred years ago. Find a puddle after it rains, and draw a chalk line around it. Where will your puddle go?