A curious patterns occurs in a group of people who all shake hands with one another. It turns out that you can predict the number of handshakes that will occur if you know the number of people. If we are in a room of 5 people, we can determine the number of handshakes by this line of reasoning: The first person will shake 4 hands (she won’t shake her own). The second person will shake 3 hands (he won’t shake his own of the hand of the first person, they already shook). The third person will shake 2 hands (same reasoning). The fourth person will shake 1 hand (that of the fifth person). The fifth person will shake 0 hands. So there will be a total of 1+2+3+4=10 handshakes