As water cools it becomes more dense.
At 39°F (or 3.98°C to be exact) water is the most dense. This is because the molecules are closest together at this temperature.
The closer the molecules, the heavier. The farther apart the molecules, the lighter.
So then if water is heavier the colder it gets, why isn’t ice more dense?
This is because the molecular structure and hydrogen bonding of the water takes up more space when it is a frozen solid rather than when it is a warmer liquid.
In other words, the molecules are farther part even though the water is solidifying into ice.
This seems counter-intuitive because as most elements solidify, they become heavier. Water is unique in this sense, because as it becomes a solid – it becomes lighter.
Think about a cup of ice water. The ice floats on the water because ice is less dense than water.
It’s also why you’d rather carry a cooler of ice than a cooler or water.
And why lakes freeze at the top rather than from the bottom.