Two sentries were on duty outside a barracks.
One faced up the road to watch for anyone approaching from the North.
The other looked down the road to see if anyone was approached from the South.
Suddenly one of them said to the other, "Why are you smiling?"
How did he know that his companion was smiling?
Although the guards were looking in opposite directions, they were not back to back. They were facing each other.
You have a sock drawer.
It has 4 black socks, 8 brown socks, 2 white socks and 8 tan socks.
You need to pull out a matching pair of socks in the dark.
There is no light and you couldn't see the socks.
How many socks you should pull out in the dark to get one matching pair of socks?
Five. You have only four different colors of socks. If you pick 5, you can surely get one pair of matching socks.