You were curious about how Castiel experienced certain things. It was early, and the motel room you were sharing with Castiel was still dark.

You, being an early riser, had opted to share with him instead of Sam or Dean because Castiel didn't sleep and wouldn't be bothered if you got up.

When Castiel had his back turned, you slapped his rear. Hard.

He didn't let out much sound. Just a yelp. He turned around, looking puzzled. "Why did you do that? Is this a human custom I am unaware of?"