xkcd Goodies: Science
Got someone who always argues with you? Science can solve your problems through the magic of the scientific method. Except perennial arguers tend not to believe proof. So good luck with that.
Science: We finally figured out that you could separate fact from superstition by a completely radical method: observation. You can try things, measure them, and see how they work! Bitches.
The graph on the back of the shirt is data from the COBE mission, which looked at the background microwave glow of the universe and found that it fit perfectly with the idea that the universe used to be really hot everywhere. This strongly reinforced the Big Bang theory and was one of the most dramatic examples of an experiment agreeing with a theory in history -- the data points fit perfectly, with error bars too small to draw on the graph. It's one of the most triumphant scientific results in history.
"Science / It works, bitches." on the front with the COBE graph on the back in white on a forest green 100% cotton t-shirt.