you'll shoot your eye out

Edgy leaves his office to happen upon Boy and Twin the Second as they are about to leave the house. Twin the Second is dressed in kickboxing protective gear and Boy has a slingshot in his hand.

Edgy: What are you doing?
Twin the Second: Boy's going to shoot the slingshot at me.
Edgy: No, he's not.
Boy: Why not?
Edgy: Because you're not.
Twin the Second: But it's fun.
Edgy: It's only fun until you shoot your eye out.
Twin the Second: That's why I'll do this. [Twin the Second puts a duffel bag over her head.]
Edgy: No. You will not fire the slingshot at your sister.
Boy: But it was her idea.
Edgy: I don't care. No.
Boy: Then I'll shoot it at this pillow.
Edgy: You will not fire the slingshot in the house.
Boy: Fine. I guess we'll go make a target outside.