Thursday

Raven spreads a new table across the Bench. This one has only water in it, but at many temperatures.

"Same substance, different numbers," she says. "What do you notice as it gets hotter?"

Rocket reads down the column. "The density goes down. Hot water is less dense than cold water."

Nova hovers over the top rows. "Look closely at the first two rows," she says. "Something odd happens near freezing."

"It goes up from 0 to 4 degrees, then down," Rocket says slowly. "Water is weird."

Raven smiles. "Water is special. That is why the lake freezes from the top."

"And why the fish stay warm at the bottom," Rocket says. Nova's light brightens. "Now read the rest."

The U.S. Geological Survey Water Science School publishes the density of water at different temperatures. Here are some of its rows.

Density is given in grams per cubic centimeter, which is the same as grams per milliliter.

Notice that the exact density of water is not really 1 gram per milliliter. It is a bit less, except at about 4 degrees Celsius.

The density of water is roughly 1 gram per milliliter. It changes with temperature, or if there are substances dissolved in it.

From 4 degrees Celsius up to boiling, warmer water is less dense. So warm water floats on cooler water.

Between 0 and 4 degrees Celsius the pattern flips. Water just above freezing is denser than water at freezing.

This is why the coldest water near freezing rises toward the top of a pond, and the ice forms there.

The ice itself is even less dense, 0.917 grams per milliliter in Tuesday's table, so it floats on the surface.

Excellent data work. Tomorrow you will find density in oceans, ponds and your own kitchen, and review the week.