He holds it out of her reach, not teasing at all, but he doesn't want her grabbing it willy-nilly either.

"See how easy it is? The first time you want to use this on another person, you remember this. Because I don't want you doing anything you regret. I want you to be safe, I want you to be able to protect yourself."

Duo clenches his teeth a little. "But I was years younger than you the first time I...hurt someone with a knife, and I'll never forget it."