Among the prime numbers there are some that are even more special. Mathematicians call them twin primes: they are pairs of prime numbers that are close to one another, almost neighbours, but between them there is always an even number that prevents them from truly touching. Numbers like 11 and 13, like 17 and 19, 41 and 43. If you have the patience to go on counting, you discover that these pairs gradually become rarer. You encounter increasingly isolated primes, lost in that silent, measured space made only of numbers, and you become aware of the distressing sense that the pairs encountered up until that point were accidental, that solitude is the true destiny. Then, just when you’re about to surrender, when you no longer have any desire to go on counting, you come across another pair of twins, clutching each other tightly.