The brain registers that food is coming, and instructs the saliva gland underneath your tongue to produce saliva, which helps break down the food as we chew it and makes it softer as it enters the gullet. As this is a subconscious reaction, it will occur even if you are not actually intending to eat the food you see or smell. As an aside, Ivan Pavlov used this reaction to illustrate his experiments on "conditioning" in dogs, but ringing a bell before feeding the dogs. When he withdrew the food, the dogs would still salivate on hearing the bell ring.