Five year old "mom, come see what I did to the car tracks".  I go into his room and the little designer had reassembled the tracks so the loop-the-loop was attached to the high spped launcher, the 4-in-1 race track reduced to a single car that was launched off a ramp into space at the end, the window sill ramp (previosly attached to the loop-the-loop) was redesigned to have a launching ramp at the end.  "Great designs buddy!"  We sat down and did an optimation experiemnt of which cars did best on which tracks to declare a winner.

5 year old: Mom, the farmers make the food for the
Publix truck, right?
Me: Yes 
5 year old: Where do they get the seeds to grow the
food?"
Me: The seeds come from their plants, like the seeds in your orange, the farmer keeps seeds from their plants to plant more food' 
5 year old: But mom, where does the FIRST seed come from  for their plants then?" 
Me: Good question, a seed store  

That was either an acceptable answer, or he was not sure I knew where the first seed came from, he gave me the nod of 'moving on'.