Prime Number 


Well, almost. A prime is also divisible by the negative of itself, and 1, but we usually don't mention that. So 29 is a prime because the only numbers that divide evenly into 29 are 1, 29, 1 and 29. On the other hand, 28 is not prime because it's divisible by lots of other numbers, including 2, 4 and 7.
Examples of primes are 2, 3, 5, 7, 11, 13, 17, ...
In around 300BC a guy called Euclid proved that there are infinitely many primes, but there are still many things unknown about them. For example, here are some primes that differ by exactly two:
One of the Millennium Problems in mathematics, namely the Riemann Hypothesis, relates to prime numbers and their distribution.
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Quotation from Tim BernersLee 