Welcome to brain Stuff from house stuff works dot com, where smart happens him Marshall Brain with today's question, what's going on with the birthday paradox? You may have heard that if there are twenty people in a room, there's a fifty fifty chance that two of them will have the same birthday. How can that be? What it really is called the birthday paradox, and it turns out it's useful in several different areas, for example, in cryptography and
hashing algorithms. You can try it yourself. The next time you're at a gathering of twenty or thirty people, ask everyone for their birthdate. It's likely that two people in the group will have the same birthday. It always surprises people. The reason this is so surprising is because we're used
to comparing our particular birthdays with other individual's birthdays. For example, if you meet someone randomly and ask him what his birthday is, the chance of the two of you having the same birthday is only one out of three sixty five or point to seven percent. In other words, the probability of any two individuals having the same birthday is low.
Even if you ask twenty individual people. The probability is still low, less than five percent, so we feel like it's very rare to meet anyone with the same birthday as our own. When you put twenty people in a room, however, the thing that changes is the fact that each of the twenty people is now asking each of the other nineteen people about their birthdays simultaneously. Each individual personally has a small chance less than five percent of success, but
everyone is trying it simultaneously. That increases the probability dramatically. The next time you're with a group of twenty or thirty people try it. You might be surprised for more on this and thousands of other topics because at housetop works dot com, the be