Final answer:
David knows what cedar smells like through the brain's interpretation of volatile compounds detected by the olfactory system and associated with previous experiences.
Step-by-step explanation:
The ability to recognize and identify smells is a result of the brain's interpretation of olfactory signals. Humans have a highly developed olfactory system that enables them to discriminate between a vast number of different odorant molecules, potentially more than a trillion. In the context provided, David would have become familiar with the smell of cedar through experience. When cedar wood is encountered, it releases volatile compounds into the air that are detected by the olfactory receptors in David's nose. These signals are then processed by the brain and associated with previous experiences of smelling cedar, leading to the recognition of this particular odor.