Body Parts

So I am up stairs in our bed room putting away some clothes and I hear Tommy utter the following sentences:

Bum Crack!
Mommy has a bum crack!
Kate has a bum crack!
Tillie Has A Bum Crack!!
TOMMY Has A Bum Crack!!!

Then I hear him run up the stairs.  He runs into our room and grabs the back of my pants and starts pulling. He gets a big smile and yells:

DADDY HAS A BUM CRACK!!!!

I am not exactly sure what happened down stairs, but some how he found out that there is a crack between two bum cheeks and that everyone has the crack and that it is called a bum crack.

Amy claims she did not teach him about bum crack....hmmmm...Tillie? Kate?

I wonder at school today if he asked Ms Monique, Ms Tanya or Ms Laly about bum cracks.