Rawson points out that although snakes can't taste, they have a primitive sense of smell. They'll extend their tongue to gather volatile molecules and then pull it back in and plug it into the vomeronasal organ at the roof of the mouth to get a reading. Snakes are keenly attuned to the aroma of favored prey--so much so that if you slip a rat's face and hide, Hannibal Lecter-style, over the snout of a non-favored prey item, a python will try to swallow it.