I just give them a blank stare and walk away shaking my head. If I am so inclined to respond, I will ask them some question about how do they do some edged tool-related task without said tool.
A side note...my roommates said that knives were dangerous and unnecessary, so, as a challenge, I took all of our kitchen knives and locked them in my room for two days. Apparently, they are useful sometimes.
My friend keeps saying
"Why do you need a knife when your teeth can do everything"
My only response has been "Because when your car flips and seatbelt gets stuck how do you plan on chewing your way out"
He refuses to take this as a valid reason...
Any ideas on how I can shut him up?