Asking students to raise their hands is a time-honored feedback mechanism in education. Hand raising allows the teacher to assess to what extent a concept has been understood, or to see where the class stands on a particular issue, and then to proceed with the lesson accordingly. For many types of questions, as the evidence here demonstrates, the tally from a public show of hands misrepresents the true knowledge or preferences of the class. The biases are predictable and systematic. Specifically, students raising their hands tend to herd and vote with the majority answer. Beyond impeding the teacher's ability to assess her class, such herding threatens to diminish learning by limiting the level to which a student engages with the questions posed by the teacher.