Flank (anatomy) Puns

I was talking to my boys about what organs they have.

"And another interesting fact, when you're born you have four kidneys but when you become an adult you end up with only two."

My wife piped in from the kitchen, "Really? I didn't know that."

"Yeah. See, (pointing to my oldest sons flank) you start with one here and one here." Then pointing at his legs, "and one here and one here. But when you grow up, these last two become adult-knees!"

She tried to groan but her laugh got the best of her.