Where do my kids get this weird streak?

"Mommy, it hurts when I do this," said my five year old. He then bent in half, twisted his upper body to the right, and pointed his right arm at the ceiling.

"Well," I said, "I can see that would be a problem. I mean, that's something people do all day long." Heavy on the sarcasm.

The older boys heard me and came to see what was happening. "It hurts when he does this," I said, demonstrating.

"And this," he said, bending in half and putting both hands on his backside.

"So you can see the problem. It's tough to go even five minutes without doing that." The boys started giggling. "But try not to," I directed him.

"Okay." They were all laughing and imitating his painful poses.

"And now," I said, "I have to go upstairs." And I bent in half, twisted sideways and pointed my right arm up. See, it's a very useful pose.