Water is so common that we rarely notice how unusual it is. Most substances denser when they , but water does the . it turns into ice, its molecules into an open that takes up space. is why ice on .

In winter, the surface of a lake may freeze while fish swim below. Because ice is than liquid water, it a layer on . This layer like a , protecting the water from the cold . Water is also at about four degrees Celsius, the water often stays at this temperature.

A water molecule is small and simple: one oxygen atom bonded to two hydrogen atoms. Yet the way these molecules interact gives water properties that are rare among liquids.

The key is polarity. Oxygen pulls the shared electrons toward itself, so the oxygen end of the molecule carries a slight negative charge and the hydrogen ends carry slight positive charges. As a result, neighboring water molecules attract one another, forming weak links called hydrogen bonds.

These bonds explain many of water’s surprising features. Because the molecules cling together, water has a high boiling point for such a small molecule and can absorb a great deal of heat before its temperature rises. This is why oceans warm and cool slowly, helping to stabilize Earth’s climate.

Hydrogen bonds also explain why ice floats. As water cools below about four degrees Celsius, the bonds begin to hold molecules in an open, six-sided arrangement. In solid ice, this structure leaves more space between molecules than in liquid water, making ice roughly nine percent less dense. The same six-sided pattern can be seen in snowflakes.

For animals living in lakes in cold climates, water’s unusual behavior is a matter of survival. If ice were denser than liquid water, it would sink as it formed. Over a long winter, lakes could fill with ice from the bottom up, and many fish and other creatures would be trapped and frozen.

Instead, ice forms at the surface and floats. The ice layer, often covered with snow, acts as insulation, slowing the loss of heat from the water below. Meanwhile, because water is densest at about four degrees Celsius, the deepest water in many lakes stays near that temperature throughout the winter.

Life does not stop in these conditions, although it slows down. Fish such as perch become less active and need less food. Some plants and algae continue to grow if enough sunlight passes through clear ice.

Winter can still be dangerous. When thick snow blocks sunlight, algae stop producing oxygen, and in shallow lakes, oxygen levels may fall so low that fish die. Scientists call this event “winterkill”.