Wednesday

Rocket lines up two cups on the sunny railing. One holds soil, the other water. "Race!" he says.

Raven checks both with the thermometer. "Same temperature to start. What do you notice about the cups?"

"Same size, same color, same amount," Rocket says. "I made sure. Otherwise it is not a fair race."

Nova settles beside them. "A fair test changes one thing only," she says. "What is the one thing here?"

"Soil or water," Raven says. "Everything else stays the same."

An hour later, Rocket reads the thermometer and his eyes go wide. "The soil won by a lot."

"Now put them in the shade," Nova says. "Which one do you predict will cool faster?"

You will plan and run a fair test: does soil or water warm faster in sunlight, and which cools faster in shade?

In a fair test you change only one thing. NASA's science fair guide calls that one thing the variable.

Only by keeping everything else the same can you be sure the variable is what you are testing.

Scientists name the parts of a test. The thing you change is the independent variable: soil or water.

The thing you measure is the dependent variable: temperature. Everything you keep the same is a control.

The National Weather Service explains that the Sun warms land and water at the same rate, but the results differ.

The Sun's rays are confined to the top few inches of soil. In clear water, light can reach hundreds of feet down.

Wind and waves also keep mixing the water, spreading the heat through a huge volume.

So it takes a great amount of heat to raise the temperature of water, and it holds that heat longer.

The result: temperature swings between day and night are much greater over land than over water.

Real data from a fair test. Tomorrow you read NASA's budget for the whole planet's sunlight.