Walking Dictionary

I have a very curious 9 year old boy. He has always asked a lot of questions - but lately his favorite has been, "What does ________ mean?" He is usually referring to a word in something he is reading, however it can come from the radio, television, a conversation he overhears, etc. I love it that he is so curious - but if I am going to be able to keep up - I think I will have to start reading the dictionary! It is so strange - there are many words that you are familiar with, and have an understanding of what they mean - but have difficulty defining.
How about you, what is the funniest, most persistent thing your children have asked?