From the mouths of babes..
Sex
When my grandson Carson was three he attended a co-op preschool.At "circle time" all the kids sat down in a circle and he announced he had something to tell everyone. The whole group got quiet, and he announced..
"My mommy only has sex with my dad because she's getting a new computer!"
My daughter was there at the time and there was not a thing she could say after that..