Mariana had played violin with the Bowie Middle School mariachi for only six weeks. The spring concert was on Saturday, and Thursday's practice was the last rehearsal.

Halfway through the second song, Mr. Cardenas raised his hand, and the music stopped. "Violins, one more time from measure twenty," he said. Then he added, "Mariana, just you."

Her face turned hot. Twelve students turned around to look at her. She lifted her bow, but her fingers felt as stiff as frozen crayons. She played the first four notes, then lost her place in the quick run of notes that followed. The sound that came out was thin and shaky.

"Again," Mr. Cardenas said, not unkindly. "Slower this time." She tried again, and again she stumbled in the same place.

After rehearsal, Mariana packed her violin as fast as she could. She wanted to leave before anyone could say anything to her. But Daniela, who had played in the group since sixth grade, sat down on the bench beside her.

"That run used to destroy me too," Daniela said. "Last year I played it so badly that Mr. Cardenas had me play it alone four times in a row."

Mariana looked up. "Really? You?"

"Really. Here's what helped." Daniela took out her own violin and played the run at half speed, then a little faster, then faster again. "Don't start from the beginning of the song every time. Just practice the hard part, slowly, until your fingers get bored."

That night, Mariana stood in her bedroom and played the same eight notes for forty minutes. Her little brother knocked on the wall twice. By the time she went to bed, her fingers knew the way on their own.

On Saturday, when the violins reached measure twenty, Mariana did not look at the music at all. Across the row, Daniela caught her eye and gave a small nod.