Water begins to evaporate whenever its molecules speed up enough to break free from their bonds. A water molecule, or H20, is made of two hydrogen atoms and one oxygen atom.
Because hydrogen atoms have a slight positive charge, and oxygen molecules have a slight negative charge, the hydrogen atoms in water attract the oxygen molecules. When this attraction is strong, relative to the speed of the water molecules, the bond holds the water molecules together as either solid ice or liquid water. When the water molecules speed up, due mainly to exposure to heat or decreasing the atmospheric pressure on the water, the bonds break and the water evaporates.