I did post the answer to this question this morning, but it is now missing! So here it is again – – Sarah Piazza answered the question correctly. Nick Giacobe did as well, but not before Sarah. I mention that, because it is an impressive question to answer:-)
The problem is actually a classic statistical calculation. More information can be found on this problem (as well as other permutations of the problem) here. But the basic solution is based on the following premise: The probability of two people not having the same birthday is relatively high, at (365*364)/(365*365). The probability of two of three people not having the same birthday is (365*364*363)/(365^3). Extending this calculation to a group of 25 people will give you the following equation:
And the final probability of no two people having the same birthday in a group of 25 is 0.4313 or 43.13%. That means that the probability of two people having the same birthday in a group of 25 is 56.87%. Isn’t that SHOCKING!?!??!!