A venn diagram with two overlapping circles. In the left circle it says "set theorists"; in the right circle it says "astronomers"; and in the middle it says "people who get excited about things money can't buy".
(The two overlapping circles also happen to look like the outlines of the Mastercard logo.)
Title text: Cantor proved that, by going along all the diagonals of a grid, you can get an infinite number of copies of the set of natural numbers from us for the price of one! Act now and you'll also get a free group with your purchase of any generating set!