Neurons are constantly performing complex calculations to process sensory information and infer the state of the environment. For example, to localize a sound or to recognize the direction of visual motion, individual neurons are thought to multiply two signals. However, how such a computation is carried out has been a mystery for decades. Researchers have now discovered in fruit flies the biophysical basis that enables a specific type of neuron to multiply two incoming signals. This provides fundamental insights into the algebra of neurons -- the computations that may underlie countless processes in the brain.