In a crowded room, two people probably share a birthday
It only takes 23 people to enter a room to give you an evens chance that two of them have the same birthday. With 75 people in the room the chances rise to 99 per cent!
The universe isn't big enough for Googolplex
A googolplex is 10 to the power of a googol, or 10 to the power of 10 to the power of 100. Our known universe doesn't have enough space to actually write that out on paper. If you try to do that sum on a computer, you'll never get the answer, because it won't have enough memory.
Seven is the favorite number
You might have guessed that most people's favorite number is 7 but that's now been proven.
A recent online poll of 3,000 people found that around 10% of them chose seven, with three as the runner-up. That might be because seven has so many favorable connections (seven wonders of the world, pillars of wisdom, seven seas, seven dwarves, seven days, seven colors in the rainbow).
Prime numbers help Cicadas survive
Prime numbers are those that can only be divided by two factors: one and itself! Cicadas incubate underground for long periods of time before coming out to mate. Sometimes they spend 13 years underground, sometimes 17. Why? Both those intervals are prime numbers and biologists now believe cicadas adopted those life-cycles to minimize their contact with predators with more round numbered life-cycles.