Greg Cashman, age 10, of Urbandale, Iowa, for his question:

WHAT DO A.M. AND P.M. STAND FOR?

Each 24 hour day begins at midnight and is divided into two parts. The first part runs from midnight to noon and is known as the a.m. The letters stand for the Latin words ante meridiem which means "before noon.''

The hours between 12 noon and midnight are called the p.m. hours. These letters stand for the Latin words post meridiem which mean "after noon.''

Astronomers call each 24 hour period of time a sidereal day. It is based on the time needed for the earth's rotation. The day actually is 23 hours, 56 minutes and 4.09054 seconds in length. The missing time is put together as February 29 every four years when we have 366 a.m.'s and p.m.'s instead of the usual 365.