I must admit I have a couple of favourites from when Cat was younger :-

The unanswerable questions -

Why don't spiders have eyelashes ?

why don't the words fall off when they roll up the television screen ?

and my personal favourite -

Cat- "What did St George fight ?"

Me - "A Dragon"

Cat - "I thought so - I told Amy it wasn't a shark !"

All the best

Julie