c: mommy, did you remember your check card?
m: yes. do you know what a check card is for?
c: yes, to pay for things.
m: but where does the money come from?
c: the bank
m: but how does the money get into the bank?
c: mommy and daddy put it there
m: that's right. but where does the money come from that we put in the bank?
c: um... >thinking hard<> is nodding in agreement, seemingly studiously listening] ...and that's how it works.
c: but mommy...
c: where do sponges come from?
clearly, i'm getting through. :)