New body parts

This morning as Cooper hopped down from the bar stool at the kitchen counter, he let out a little yelp and reached back to grab one of his shoulder blades. Poor guy must've bumped it. Cooper grimaced and as he came to me for comfort, he mumbled that he hurt a body part on his back. I didn't catch what he said the first time so I asked him to tell me again and more clearly he said, "I hurt my back boob." Where do they come up with these things?? I informed him there are no boobs on his back, just shoulder blades and that 'boob' isn't a nice word. We don't even allow it in our house, but fortunately his use of it seemed pretty innocent. That was definitely a first. I hope it's the last.