Four color theorem states that any given plane figure separated by lines can be colored using not more than 4 colors!
First proposed in 1852 when a man named Francis Guthrie who was coloring a map of the counties of England discovered that he was able to color all the adjacent countries using only 4 colors where none of the adjacent countries had the same color.
Even though it worked wonderfully it was hard to prove why it happened.
Take a look at the below map and count the number of colors being used.
Many mathematicians tried to prove this theorem but none were ever able to prove it until 1976 when two mathematicians Kenneth Appel and Wolfgang Haken from the University of Illinois proved this using computer. This is believed to be the first theorem to be ever proved true using a computer.