Water has its maximum density at

Explanation

Cooling water from room temperature increases density as normal, but below 4 degrees the open hydrogen bonded structure begins to form and the density falls again, so the maximum occurs at 4 degrees. This is why the bottom of a deep lake stays at about 4 degrees while ice forms at the surface, allowing aquatic life to survive the winter. At 0 degrees water is already less dense than at 4.