Answer: A birthday
In the field of probability theory, there is a problem known as birthday paradox, regarding the probability that in a selection of N randomly selected persons, some of whom have their birthday. This probability reaches 1
It is fascinating, however, how quickly the probability of sharing a birthday increases. In a group of 23 people, about the size of a small class, the likelihood that two people have their birthday has already risen to 50 percent. To reach 99 percent, you only have to put together two classrooms. In a group of 57, there is a 99 percent chance of having a birthday together.
While this looks like a mathematical salon trick, the math behind the birthday paradox has actually been successfully used as a well-known cryptographic attack, the Birthday Attack, which uses probabilistic models to reduce the complexity of cracking hash functions.