This is an improved variation of the "Cup on Car" prank. Use the higher-quality shopping bags (I found the Walmart type tear too easily). Double bag, with the inner bag stuffed with newspaper and tied. I've gone down the Interstate with this on top of my car and it stays secure..
Step 1: The Key Is the Magnetism!
Drop two large magnets inside the double-bag before stuffing and tying, then click it on the roof of your car. CAUTION: Will not work on convertibles or Corvettes! I had one lady follow me for two miles, flashing her lights and honking...I finally pulled over and gave her a dollar for being a Good Samaritan. Have fun!
I've had a blast this morning driving through the rush-hour traffic! April Fools! Hahahahahahaaaa!