From an email I sent my friend after watching her kids yesterday (this refers to my 3.5yo and her 2.5yo.)

I regret to inform you that Zeff told his first off-color joke today and that your son, Ben laughed at it.
He called the "apple machine" a "diaper machine". And Ben thought it was hysterical and repeated it several times.
I thought it was an important milestone for both of them in their humor development, and that you would want to know.
:)
Katy
Those kids are way too funny, sometimes.