Tony Erb, age 15, of Dewitt, Mich., for his question:

WHAT IS A QUARK?

A quark is an atomic particle that exists only in theory.

Many scientists say that all the known atomic particles may be combinations of three basic bits of matter. They call these supposed bits of matter quarks.

Scientists have tried to prove the existence of quarks by producing them in particle accelerators. They also study cosmic rays in the hope of identifying quarks that may exist deep in space. But, as yet, no experimental evidence of the existence of quarks has been found.

 The    idea of the quark came to scientists with the discovery of many

kinds of atomic particles. Particles that once were thought to be

basic were found to decay or break down into smaller particles.