Safety First!

The boy has requested something new for his jokes. He wants the word that makes the joke funny underlined. I'll be highlighting the puns from now on. Today's was tricky though, because I feel like two of the words could be used as puns here.

I underlined 'pin' as I feel it's the punniest choice here, but safety could work too since most weapons have them. I'm no grenade expert, so I'm not sure if grenades have a safety on them or not. I would guess that's the pin's job.