The Birthday paradox is that in a group of only 23 people, there's a greater than 50% chance that at least two people share the same birthday.
Here is an app that calculates the likelihood that at least 2 people in a room will have the same birthday. Using random number generation, the app lets you choose the number of simulations ran and the number of people in the room, as you increase the number of simulations you get closer to the "True" probability, but will take longer to run.