Where does he come up with these?

So as I've mentioned before, Austin is finally potty trained. This is such a wonderful thing for us. Now we only have one left in diapers!

However, we still put pull-ups on him at night. He is getting better about waking up during the night and going and/or staying dry. But we still have the pull-ups so an accident does not cause a set back.

Friday night at bedtime we had the following conversation:

Me: "Austin, go take off your underwear and put them in the dirty clothes. Then bring me a pull-up and we'll get you ready for bed."

Austin: "Okay Mommy!"

He runs off and a few minutes later comes back in the room. In one hand is a pull-up. The other hand is playing with himself. Now I know this is normal for boys, but I don't necessarily want him doing it in front of everyone.

Me: "Austin, honey. Don't be doing that in front of people, okay."

Austin (with a very serious look on his face): "But Mommy, it's my tail! It's my potty tail!"

Where in the world does he come up with these things! And what do you say to that? The best I could come up with was - "I know, honey. But don't play with your potty tail in front of other people, okay."