How do you explain patents to a non-lawyer?

First a little background: I am a graduating 3L law student going into IP law (Biology B.S. and M.S.), about to head to Chicago to start my patent/(c)/TM career. I have a lot of non-lawyer friends plus a lot of non-ip lawyer friends; With the rising visibility of patents within the wheelhouse of those that may not have thought about them before (thanks, America Invents Act and Google/Apple/MS/Samsung etc), they have come to me to explain it all to them. While I have tried (selfless blog plug), they continue to see it as just "getting in the way of innovation."

Explaining it as "getting in the way of copying" does not seem to do the trick either. So my question to you; When someone approaches you, ignorant of how the system works, how do you explain the way that patents function in "promot[ing] science and the useful arts"?