Donimarie Chapman, age 8, of Milwaukee, Wisconsin, for her question:

What do a.m. and p.m. mean?

A day on the calendar has 24 hours. But the face of a clock has only 12 hours. It must tick around once to mask the first 12 hours and around once again to mark the next 12 hours in a full day. Suppose someone says that the train leaves at 2 o'clock. If we want to catch this train, we must know more about the time. We must know if it leaves at 2 in the early morning or 2 fn the afternoon. So we add two little letters. The morning train leaves at 2 a.m. The afternoon train leaves at 2 p.m. In a full day, there are twelve a.m. hours and twelve p.m. hours.

The letter "M" tells where the first twelve hours ends and the second twelve hours begins. The letter "M" in both a.m. and p.m. stands for "meridian." And "meridian" means noon or midday. It shows that the first twelve hours of the day ends at noon and the second twelve hours begins at noon. The letter "A" in a.m. stands for "ante," an old word that means "before." The a.m. hours are the twelve ante meridian hours. They begin at midnight with the new calendar day. And they last through the morning until 12 o'clock noon. The letter "P" in p.m. stands for "poste," which is an old word for "after." The twelve p.m. hours are the poste meridian hours that begin after noon and last until the day ends at midnight.