The materials you need are simple:
1. A coffee cup that you don't care much about
2. A fairly strong magnet. I'm going to use a hard drive magnet
3. Clear packing tape
4. Your car
5. A trip to the drive through at McDonald's, Burger King, or Dunkin Donuts, etc
Step 1: Make It
Turn the coffee cup over and place the magnet in the center on the bottom of the cup. The magnet keeps the mug in place on your car while you drive.