Matthew Holden, age 11, of Marquette, Mich., for his question:

WHY IS A NAIL MEASURED IN PENNY UNITS?

Nails are measured in units called pennies, designated by the letter "d." Nails range in size from 2 penny nails, that are one inch long, to 60 penny nails six inches long. Nails that are shorter or longer than those are measured in inches or centimeters.

The system of measuring nails in pennies probably started in England several hundred years ago. No one knows the source of the penny unit.

Some people say a penny once stood for the weight of 1,000 nails. As an example, 1,000 4d nails weighed four pounds.

Others believe the unit represented the price of 100 nails, so that 100 4d nails would have cost four pennies.

Nails actually go back to ancient times. The earliest were made about 5,000 years ago in Mesopotamia. Artists used them to fasten sheets of copper to wooden frames to make statues.