Water covers three-fourths of the planet's surface. Polar ice caps contain most of the world's fresh water in the form of ice and snow. After nitrogen and oxygen, water is the third most abundant substance in the earth's atmosphere. Yet, as common as water is, it is one of the strangest chemicals known. Unlike most other liquids, water flows faster under high pressure than under low pressure.
Ice is unusual because it floats on liquid water.
Unlike most other solids, ice melts under pressure. The melting and boiling points of water are unusually high for its molar mass. Water's uncommonly high specific heat helps to keep the planet warm and its high heat of vaporization stores solar energy that is released in storms. Read Wet, Wild and Weird , a Discover magazine article on the strangeness and simplicity of water, and find at least three unusual properties of water. Find out at The Straight Dope .