Well first, you could talk to them about the bone structure of snakes, and that they actually have alot of bones (alot of people think they don't have any). Then, you could talk about the constriction, and demonstrate by wrapping him around your neck (if he isn't too big). Then you could talk about their heat pits, and how they sense their prey. Then you could tell them that they are harmless to human, and let each one who wants to pet, or hold it for a brief time.