There is a door, on the side of the door that you are on, there is a skeletal army ready to trample you underneath their feet, on the other side is a city with a wall that will keep you safe. You need to get in the door but there is some sort of code to get into safety; one man goes to the door, and the doorkeeper says "eight" and the man says "five" he lets him in. The second man goes up to the door and the doorkeeper says "nine" and the second man says "four", and he lets him in. Finally, it's your turn, you only get one chance to guess. The doorkeeper says "twenty one". What do you say to get to safety?
You would say "nine" as, 21 is nine letters long. It's quite simple actually. Eight is five letters long hence "five" and nine is four letters long hence the word "four".