William wouldn't show his eyes.

The guarded, gaunt 10-year-old gazed at the floor of Dr. Irwin Redlener’s mobile pediatric unit parked in his Brooklyn neighborhood, answering the pediatrician's questions in monosyllables.

Then Redlener, who dreamed up the mobile unit — a big blue bus — with his wife, Karen, and singer-songwriter Paul Simon to bring health care to the inner-city poor, asked William what he calls “the big question”: What do you want to be when you grow up?