Have you ever gone outside to splash in the puddles after a summer rain shower, only to find that the puddles have disappeared an hour later? Or hung up a wet towel, only to find that the towel is completely dry by the next morning? Where did all of the water go? An important scientific principle is the conservation of matter. It states that the amount of matter in a closed system will remain constant over time, which is another way of saying that basically matter is neither created nor destroyed.
The principle of conservation of matter has since been expanded to the conservation of mass-energy after Einstein theorized that matter could be converted into energy and energy could be converted into matter (E = mc2) in 1905. But this revision is usually only important when you are working with nuclear reactions, in the vicinity of a black hole, or moving close to the speed of light.
So, if matter can neither be created nor destroyed, then water cannot simply disappear… it has to go somewhere. Let’s take a closer look at the water in our puddle and see what happens over time.
What we call water is actually a collection of water molecules. A water molecule  is also known as an H2O molecule because it consists of two hydrogen atoms and one oxygen atom chemically bonded together. Water molecules can exist in three different states: solid, liquid, or gas. The water molecules in the puddle or on the towel are in a liquid state. However, over time, individual water molecules break away and enter a gas state. When liquid water dries, the water molecules are breaking away from the liquid state and forming water vapor in the air. This process is called evaporation.
You will learn much more about atoms, molecules, states of matter, and evaporation later in this unit. For now, all you need to remember is that water is made up of water molecules, and that matter is neither created nor destroyed… it is conserved.