Where do they learn this stuff?

I was working in a second grade classroom this afternoon.  The kids were working on their center work and I was with a reading group.  All of a sudden, a little boy came up to me and said, "Mrs Emily, can I talk to you in your ear?" I said, "of course" and leaned closer to him.  He had been working at a making words center where you move around the letters of your name and make as many words as you can.  He said, "Can I write ass on my paper?"  I tried to contain my laughter as I told him no because we don't use words like that at school.  This child doesn't write too many words correctly but he sure knew how to spell ass correctly.  What a day!