A bite into a lemon wedge sends citric acid molecules and other compounds to the tongue and through the nose, a one-two punch of the sour taste and citrusy scent that the brain senses as lemony. The tongue has dedicated receptors for detecting molecules that correspond to each one (SN: 1/25/16). Smell is more complicated, with about 400 types of smell receptors in the nose that capture signals from many, many different molecules.
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