Mary Ballard, age 14, of E1 Paso, Tex.. for her question:

WHY DO WE HAVE LEAP YEAR?

A leap year has 366 days instead of the usual 365. The extra day is added about every four years to the month of February, giving that month 29 days instead of its usual 28.

Leap year is part of the Gregorian calendar.

Leap years were added to the calendar to make each calendar year nearly the same as the solar year. A solar year is the time required for the sun to pass the vernal equinox twice.

Actually, leap year doesn't happen every four years. It occurs only in each year that can be divided evenly by four, except the years that mark the even hundreds, such as 1500. The only century years that are leap years are those than can be divided evenly by 400, such as 1600 and 2000.