Water is found in three forms - liquid, gas, and solid.
Liquid is the state with which we are most familiar. As water molecules heat up, they start moving faster and faster. Imagine the molecules like popcorn - You are popping popcorn in a pot on the stove. First all the kernels lie together in the bottom of the pan, but as you increase the heat, they start jumping around, faster and faster, going away from each other until they hit the sides of the container. Popcorn jumps out of the pan and into the kitchen.