This great upload by numberphile covers the football pitch birthday probability problem. In essence, the probability of sharing a birthday with someone in a football game (including the ref) is more than 50%. Weird but true. You can make a whole lesson out of this, (hint: use Google Calculator for the maths bit, as a normal calculator will die trying), then finish the lesson asking everyone in the class to declare their birthday. Chances are, there will be two the same!