Following the rules:
There are always at least a few water molecules in the air. These molecules are so small that they are usually invisible to the human eye. When a bunch of these water molecules begin to group up water drops begin to form.

The most important rule that water molecules follow in creating water drops relates to dew point temperature.

Dew Point is the temperature at which condensation takes place.

Condensation is the process of water molecules grouping up and changing to liquid to form drops of water.

Dew point is also considered as a measure of moisture within the air. During the summer, when it feels "sticky" outside, it is because the dew point is very high (a lot of moisture within the air).