Boys. They just innately know the things that are gross.
Whats more, they find them hilarious.
How? We never taught our son to laugh like a hyena when he *cough, cough* passed gas. Nor did we teach him to grin after he belched, giggle when someone said 'panties', or squeal with glee when we told him he was filthy.
He just does.
So it should have been beyond me to be shocked when he put his nose right over the pumpkin stem when I was snapping pictures that he requested be taken.
Then announced the pumpkin was a giant booger from his nose. And laughed like a maniac.