One of the most striking peculiarities of water is that it's less dense as a solid (ice) than as a liquid. This is unusual, as most substances become denser when they freeze.

Here's why this is important:

* Ice floats: This allows aquatic life to survive in cold climates because the ice forms on the surface, creating an insulating layer.

* Water expands when it freezes: This can cause pipes to burst and can be a factor in weathering rocks.

* Water is a good solvent: This allows it to dissolve many substances, making it essential for life and transporting nutrients.

This peculiar property of water is due to the way its molecules are arranged. Liquid water molecules are closer together than ice molecules, which are arranged in a crystal structure that creates more space between them.