You'll want to test your mental powers, today being Wednesday. You can do so with this question from Marcus du Sautoy:
At [a] party with 100 guests, everyone shakes hands with everyone else [he means just once with each person]. How many handshakes take place?
The answer is given at the bottom of du Sautoy's column. But just hang on. The real test is yet to come. Here it is. Suppose I now ask you how to get to the answer by another method than the one he gives. Can you do it? No prizes for emailing me with your solution, though I will be glad to get your email all the same.