My older son, B2 ("Blondie Boy") is very earnest.
Last night the family was watching t.v., and there was a characater smoking. I turned to my kids and said, "You guys aren't allowed to smoke".
Blondie Boy replied, "Un-less you are ninety-nine years old. And then you can."
This is the same kid who, when I warn them of safety things -- like "Be sure to hold my hand when crossing the street" -- will say, in a matter-of-fact voice "Be-cause then a car will hit you. And then you will die."