Why does it rain?

The rain is really coming down at our house right now. We were just eating lunch and Mackenzie asked "Why does it rain?" I replied with "the clouds have a lot of moisture so it falls out of the clouds as rain". I thought this might be a good answer but then came "what is moisture?" So I figured we would have to use the mighty internet and get this curious 6 yr old some answers. She has had several other questions but I think we have given her enough answers.
A-evaporation
B-condensation
C-precipitation
D-collection