Party Crashers

At a party, the host was getting worried because there were too many people and not enough refreshments. He was sure that not all of these people had been invited but didn't know how to tell which ones were the crashers. Then he got an idea....

He turned to the crowd of guests and said, "Will those who are from the bride's side of the family stand up please?"

About twenty people stood. Then he asked, "Will those who are from the groom side of the family stand up as well?"

About twenty five people stood up. He then smiled and said, "Will all those who stood please leave? This is a birthday party.