My baby brother was born when I was twelve, and my mum had post-partum psychosis and was in hospital a lot, so I looked after him mostly until he was one or so. He was a bit clingy, and used to cry if I didn't carry him around. When he was about nine months old I wanted to go do something (can't remember what nearly twenty years later), so I propped him up on the piano stool (he could sit up fine) and left him happily bashing away. Couple of minutes later I hear this almighty scream, and dash back to find him flat on the floor, where he'd obviously wriggled and fallen off backwards landing on his head. I haven't even told him about it, but he just started uni, so I don't think I damaged him permanently. He never did like playing the piano afterwards though.