Density of water at 4 degrees celsius?

1 gram per cubic centimeter

4 degrees is chosen because that is the maximum density of water.

The temperature scale is Celsius, named after Anders Celsius, an astronomer.

"Maximum density" means that above 4C, warm water floats on top of cold, and below 4C, cold water floats on top of warm. So every fall, all the lakes roll over, carrying oxygenated water to the bottom to support fish while the top is frozen. In the spring the lakes all roll over again, carrying food for bugs which the baby fish eat. If it were not so, all lakes would be frozen at the bottom and dead at the top.