Willie Clark, age 9, of Keene, Ontario, Canada, for his question:

Why are some years called leap years?

A leap can take you forward to catch up with the times. This is what the calendar does when we add an extra day to a leap year. We measure the calendar by the time it takes the sun to orbit once around the sun. We want to have a whole number of daily dates in the year, but the earth refuses to go along with this neat scheme. It takes about 365 1/4 days to orbit the sun. This gives us 365 days with a quarter day dangling at the end of the year.

Long ago, the calendar had 365 days and the quarter day was ignored. But each year it lagged farther behind the earth's yearly orbit. Finally it had to leap ten days ahead to catch up. So it was remodeled to stop this from happening again. The new plan was simple because in four years the quarter days add up to a whole day. So for three years we give our calendar 365 days. The fourth year we give it 366 days. It uses that extra day to leap forward to catch up with the times. This is why we call it a leap year and the leap day is added to February.