I love those replies, comes back at you as well, my 4yr old son is always amazed, how do I know he has hidden something behind his back and I know what it is, he is suprised I know he has broken something etc etc. So I get, how do you know Mom? Do you know everything, so I say yes in a warning he can't get away with anything.
Then when we are out and he says, why is that, or why are they going there? I say I don't know!
Silly me............I get the reply. So you don't know everything then do you Mom? GGrrrrrrr