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.

Step 2:

Take a piece of packing tape and tape the magnet to the cup. Make sure that the tape is on very well by running your finger all around the magnet and where ever the tape hits the cup. You'll probably want to use multiple pieces of tape in order to keep the magnet in place.

Step 3:

Turn the cup over. Mine doesn't even look like there's a magnet underneath. Yours doesn't have to look like this but, it's less noticeable.

Step 4: Go to Town

Place the cup on your car and take a trip to a coffee shop. You're going to get a ton of weird looks so be ready. It's really hard not to laugh but the best part is, they don't know that you know that it's up there!

NOTE***** Don't drive 80 miles an hour down the highway, the mug will fly off the the road, someone will swerve behind you, they'll hit the gaurd rail, and die, so please drive slow, like a max of 45! If you have to drive a while to get to town or the speed limit is high before you get to town, pull into a parking lot and put the cup on your car then.