“Tarbooz”, my teacher exclaimed loudly at dinner the first night at camp in Tajikistan. I looked up surprised, not having heard the word in years. I experienced an overwhelming feeling of déjà vu. Neuroscientists say that déjà vu occurs when there is trouble reconciling firing memory neurons with sensory input (though, admittedly, the mechanism is not well understood). Then it hit me: the image of my mother, holding a plump watermelon, and murmuring the same word I just...