## Where Did the Water Go?

Mary Alice was annoyed. The watering can had water in it when she left it on the window sill on Friday, but it was empty on Monday morning. She didn't have time to fill the can before class started, and, as soon as Ms. Wilson began the class, she raised her hand.

"Who used the water?" Mary Alice demanded. "Did someone drink it? Or spill it?"

No one had touched the water since Friday. Ms. Wilson realized the class had a science question to solve. "What do you think happened to the water?" she asked.

Jennifer had an ingenious explanation. "I bet Willie the hamster got out of his cage and drank it. We could prove it by covering the can and see if the water level goes down the next day."

"I don't think Willie can get out of his cage," said José. "Let's figure out a way to know what he does at night. Maybe we could put his cage on the sand table and see if he leaves footprints."

Before leaving, the class covered the watering can and smoothed down sand around Willie's cage. The water level remained the same and no little paw prints appeared in the sand.

"But wait," said Kahena. "Why should Willie get out of his cage? He can see the can is covered. Let's leave it uncovered and see what happens."

So the class again left the cage in the middle of the sand table but left the cover off the can. It took several days for the water level to drop, but it did go down, and there were still no footprints in the sand. By this time, the children were willing to let go of their original idea about the water's disappearance, and Ms. Wilson suggested an alternative experiment: "Let's put a jar of water in the window sill and measure it each day with paper slips to see if we can learn anything from the water's changes," she said.

After several days of observation, the students saw a pattern: the water was falling steadily but did not decrease the same amount each day. When they tried a differently shaped container, the rate changed. As the children worked toward developing an understanding of the influence of surface area and air temperature on evaporation rates, they discussed and predicted results of experiments they designed themselves.

This scenario has been adapted with permission from the Draft National Science Education Standards.Copyright 1994 by the National Academy of Sciences. Courtesy of the National Academy Press, Washington, D.C.

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