You can't see molecules even with the strongest microscope, but your nerves can detect them. Sugar and salt molecules make nerves in your tongue fire. Molecules in the air reach your organ of smell and make your smelling nerves fire. When a nerve fires, a message travels up the nerve to your brain. It's the brain's job to recognize the smell so you know how to react to it.
Judgments based on false perceptions can be errors.
Cobb uses exercises that show how all our senses are capable of being fooled. In one exercise, by plunging your hand, encased in a rubber glove, into cold water, your hand mistakenly feels wet. Repeating the experiment using warm water results in little or no illusion of wetness. In the first instance, a gloved hand in cold water would feel both these sensations, though the same hand in warm water would only feel the pressure. Cobb is very adept at providing clear, brief explanations of the scientific principles involved.