Why does your hair stand on end when you rub a balloon on it, anyway? The usual answer from science class is that electrons are transferred from your hair to the balloon, leaving your hair positively charged. The charged hairs repel each other and stick out. Except . . . why do electrons get transferred from the hair to the balloon? Why don’t they go the other way? That’s a great question, and the answer is that no one knows. Physicists don’t have a good general theory for why some materials shed electrons from their surfaces on contact while other materials pick them up. This phenomenon,
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