I have put 2 cars on the screen and they are moving just fine.  What I can't figure out is why they don't leave an image on the screen as they move?  What makes them erase?  I can't see any code--there's not much--that tells the program to erase the previous image.  So I'm wondering why they erase.  Maybe they don't erase at all, but I can't figure out why it acts how it does.

Any help here?  This could be very simple, I think.

Thanks.