"Baking a Cake" joke

One day a little boy and his mom were walking down the street, and the little boy noticed two birds doing "it" in their nest.
"Mommy, what are those birds doing?"
"Oh, they're baking a cake, that's all."
Later on they went to the zoo, and the little boy again saw two orang-utans doing it in the monkey pit, and again, "Mommy, what are those monkeys doing?"
"They are baking a cake too!"
Later that night, the little boy went to sleep, but was woken up in the middle of the night by a loud scream, followed by moaning.
The little boy, confused as hell, decided to go investigate. He walked in his mom and dads' room and noticed a weird act going on in their bed, and decided to run back to his room and go back to sleep.
THE NEXT MORNING...
"Mommy," said the little boy, "Were you and daddy baking a cake last night?"
"Yes, son, we were,"
"Okay, good, cause I went in your room this morning and licked the frosting off the sheets."