Thanks to psycho-boy, the one-note chorus has started chirping again, asking their thick-witted questions. LawDog has handily demolished the "Why don't we license guns like cars?" line, but I'd like to expand on that one with a little thought experiment I picked up from Marko back in the day.
When someone asks you about licensing and registration, pick up a pen and a sheet of paper. Tear the paper in half and hand half to your questioner. Say "Okay, this pen is a gun. The paper I'm holding is my license and the paper you're holding is the registration. Using only these two pieces of paper, explain to me just how you are going to keep me from shooting someone?" After the initial red-faced sputtering, the responses are invariably hilarious.