Imagine you have a set of 3 dice, red, green and blue. You pick a die and then I pick one. I can guarantee that whichever one you pick, I can choose one where I'm more likely to get a higher number than you when we roll them!
These dice are called NON-TRANSITIVE, which means that even though blue usually beats green, and green usually beats red, red still usually beats blue!
The numbers on the dice are: Blue 3 3 3 3 3 6, Green 2 2 2 5 5 5, Red 1 4 4 4 4 4.
You can work out the probabilities of Blue beating Green etc, they are 21/36, 21/36 and 25/36, so you don't always win, but it's better than 1/2 each time.
These aren't the only dice that this trick works for, here's ace mathmo James Grime to explain some more. You can buy the dice from his website at www.mathsgear.co.uk.