A metronome ticks time. Not for the student, but for the teacher, who plays a short piano melody. Without missing a measure, the student follows with an improvised, yet derivative, cello run. The student plays the same run again, and then again. \u201cI have it looping, actually, so you can hear the response over and over again,\u201d says the teacher, Jesse Engel, a computer scientist with Google Brain.