Mr. Mitchell: Here's a question for you. Why does February sometimes have twenty-nine days? It comes down to how long it takes Earth to travel around the Sun.

Mr. Mitchell: We say a year is three hundred sixty-five days, but the real number is closer to three hundred sixty-five and a quarter. That extra quarter of a day doesn't sound like much, but it adds up. After four years, it's almost a whole day.

Mr. Mitchell: If we ignored it, the calendar would slowly drift away from the seasons. Over hundreds of years, the months would move out of their usual seasons. So about every four years, we add one day, February twenty-ninth, to keep the calendar in line with Earth. That's a leap year.