"What are joints mommy?"
"Uhhh" (this is the sound I make when I'm thinking about how to explain something to a 4yr old--it happens a lot) "They are the parts inside your body that bend, they connect your pieces of bones together." (???)
"Like my fingers?" he asks as he bends and unbends his fingers.
"How many joints are in your fingers?" I ask him.
He and dad count "1, 2, 3, 4...", as they bend their fingers, opening and closing their hands. "15!" they decide.
"What if you bend all your joints at the same time!?" Mr. Baseball thinks out loud.
"Try it!" I say.