Would you like to smile for 10 seconds? Read this short story:
*Story begins*
During World War II, numerous fighter planes were getting hit by anti-aircraft guns. Air Force officers wanted to add some protective armour/shield to the planes.
The question was "where"?
The planes could only support few more kilos of weight. Mathematicians were called for a short consulting project.
Fighter planes returning from missions were analysed for bullet holes per square foot.
They found 1.93 bullet holes/sq. foot near the tail of planes whereas only 1.11 bullet holes/sq. foot close to the engine.
The officers thought that since the tail portion had the greatest density of bullets, it would be the logical location for putting an anti-bullet shield.
A mathematician said exactly the opposite; more protection is needed where the bullet holes aren't - that is -around the engines.
His judgement surprised everyone. He said "He said We are counting the planes that returned from a mission. Planes with lots of bullet holes in the engine did not return at all".

Moral: Not everything that counts can be counted, and not everything that can be counted, counts.

Source: From the book -
"How Not To Be Wrong", by Jordan Ellenberg.