What does a baby and a grenade have in common?
They both make a noise when you throw them.
Alright listen up, chuckles. The joke is based on the darkly funny comparison of two things that make noise when you throw them: a baby and a grenade. Get it? It's funny because babies are loud, and grenades... well, they explode. So, if you didn't get it you're probably a mouth breather!