Pond Boss Forums INTRO Questions & Observations Aeration "Supercooling"

Water is the densest at 39 deg F (4 deg C). It's been a long time since college Physics & Chemistry, but I will try to remember enough to give a better explanation than "Just because."
Liquid water is just that, a liquid. There is no structure between any of the water molecules and their spacing, while much tighter than in the gaseous state (steam), is controlled by how much energy they have and therefore how fast they are moving. The energy is the temperature; the warmer the water, the more energy the molecules have, the faster they move, the farther apart they are. Warmer water means less dense (lighter in weight).
Colder water means less energy, slower molecules, tighter spacing, more dense (heavier in weight). So normally the colder (heavy) water sinks and the warmer (lighter) water rises, which causes the normal Summer temperature gradient that may be seen all year round down South where "Ice" comes out of the freezer.
But when water gets cold enough, it freezes. Ice is a solid, which means the water molecules in it are rigidly locked into position. They have to be lined up in those 60 & 120 degree angle formations that make the characteristic 6 sided figures we know as snowflakes.
Now comes the tricky part. Liquid water doesn't freeze into a solid until it's temperature drops to 32 deg F. But when the water temperature gets below 39 deg F, the molecules start to form up into the 60 degree angle formations. They don't actually get all the way there until 32 deg F, but they start to line up, kind like being tied together with bungee cord as opposed to being rigidly glued together like they will be once they're ice. This semi-structure forces the water molecules farther apart than they were just above 39 deg F, overpowering the closing-up effect that lowering temperature was having on molecule energy and spacing.
So from high temperatures down to 39 deg F, cooling water packs it's molecules tighter and makes it denser. Below 39 deg F, (partial) molecule structure overcomes the energy loss packing, and the molecules grow farther part, making the water less dense. At 32 deg F ice forms, which is less dense (lighter) than water, so it floats. This gives the (northern) winter temperature gradient with the densest, warmest (39 deg F) water on the bottom, the lighter, colder (32-39 deg F) water above that, and the lightest stuff (ice) on top.
If this weird condition didn't occur and ice sank, not only would your pond freeze from the bottom (as pointed out above), the oceans would, too. Studies have shown that they would also stay frozen except for a meltwater layer on top, tying up most of the planet's water and making life on Earth very different, if not impossible.
If there's a real (or just better) physics guy on the forum, you can straighten this out, but that's how I remember it.