Recurring Conversation

I've had a variation of this conversation at various times with the older 4 boys over the past few years. Most recently it was with J when he felt the need to touch an empty bullet casing to his lips (the bullet casing was from a Memorial Day service).

Concerned Boy: "Mom? If I [lick, touch, eat] [whatever the item in question] what'll happen?"
Me: "Probably nothing"
Concerned Boy: "Would it kill me?"
Me: "Probably not. Why?"
Concerned Boy: "Because I just did."

Explain this to me--if an alarm bell goes off in the boy's head after he does this thing why does he not have the same alarm bell go off before he does it?