How many people need to be in a room to guarantee that at least two people will have the same birthday?

To guarantee that at least two people in a room share the same birthday, you need at least 23 people. This is based on the "birthday problem," which considers the probability of shared birthdays among a group. With 23 people, the chances of at least one pair sharing a birthday exceed 50%, due to the limited number of days in a year (365).