Handshaking
Riddle: If seven people meet each other and each shakes hands only once with each of the others, how many handshakes will there have been?
Answer: Twenty one. Most people would think there were 42 handshakes. The first person shakes the hand of 6 others, the second person shakes the hand of 5 remaining people, the third person shakes the hand of 4 remaining people, the fourth person shakes the hand of 3 remaining people, the 5th person shakes the hand of 2 remaining people and the sixth person shakes the hand of 1 remaining person. 6+5+4+3+2+1=21
Source: https://www.riddles.com/679
