Merritt Brown, age 16, of Tucson, Ariz., for his question:

WHEN DID THE MAYO CLINIC START?

In Rochester, Minn., is one of the world's largest medical centers. It is called the Mayo Clinic, and there a staff of physicians care for patients through an integrated group practice of medicine.

On the Mayo Clinic staff are about 500 doctors, with another 650 younger physicians assisting the staff as part of their residency training in medical and surgical specialties.

Dr. William Worrall Mayo and his sons William James and Charles Horace started the clinic in 1889. The name Mayo Clinic dates from about 1903.

Just before World War I, the brothers turned the clinic into a general medical center. Now about 225,000 patients are seen each year with about 3 million patients having been cared for since its doors opened many years ago.