I'm tickling my bum

So... this morning, Reilly, who is currently being potty trained comes in my bedroom with his hand in the back of his boxer shorts.
I said "Reilly, get your hand out of your pants"
"it's okay Mom"
"why is it okay"
"because it's not through"
"it's not through what???"
"it's not through my underpants, it's just in my shorts"
"well.... what are you doing"
"I'm tickling my bum"

I suppose a light went on. The light that showed me that in the world of a 3 year old, tickling your own bum is perfectly acceptable. As a matter of fact, it's a good time.