'Fess up. Who told him about skateboards?

Rigby is on a skateboard kick. I guess we've seen a couple pass by in parking lots, but the fascination is untraceable. When we walk down a hill, he begins extrapolating on how a skateboard would move on this sidewalk and what would be the best path to take.

He wants one.

I told him that he's too small.

He wants a small one.

Then I told him that he has a scooter, which is the small version of a skateboard. So he wanted to ride his scooter to the bus stop to pick up Bethelle. He rode some and walked some. At the end of the walk is a short, steep hill. He had me hold his hand while he stood sideways on the scooter, and we rode it down the hill like a skateboard. Where does he come up with this stuff?