The "birthday problem" is often referred to as the "birthday paradox" due to the apparent contradiction between the relatively small number of people needed in a room to have a relatively high probability of two of them sharing a birthday. Another synonym for the "birthday problem" is the "birthday coincidence problem," as it deals with the probability of a coincidence occurring. The term "birthday puzzle" is also used, as the problem is often presented as a puzzle or brain teaser. Finally, it is sometimes called the "birthday paradox" because it seems counterintuitive that such a small group of people would have a high probability of sharing a birthday.