My son won't stop smacking his penis like he's angry at it.

My son is 18-ish months. Every time I take his diaper off to change it or to bathe him, his first instinct is to repeatedly SMACK THE LIVING HELL out of his penis, repeatedly. Like... Smacking it so hard that if he were to hit his sister with this much gumption, he would be in trouble.Is this a normal thing to do? I worry that he's going to hurt himself. And ideas on how to get him to stop? Or will he just grow out of it? via /r/Parenting https://ift.tt/2vZfnQ2