Kristie Humphrey, age 12, of Winston Salem, N.C., for her question:

HOW DOES A SMOKE DETECTOR WORK?

A smoke detector is a unit installed in a building that gives a signal in case of a fire.

In a growing number of locations today, building codes are requiring new homes and existing apartment buildings to have smoke detectors installed.

Detectors are classed either as photoelectric or ionization. In the photoelectric model a small light remains on continuously and any smoke entering the unit reflects the light to a photocell, setting off an audible alarm. In the ionization type, an extremely weak electrical current is formed by the ionized molecules in a minuscule amount of radioactive material. If smoke interrupts the flow of ionized molecules, the resulting drop in current will sound the alarm.