Generally speaking, training (or learning) generally involves the use of an antecedent (an external cue or stimulus), a behaviour from the individual concerned and a consequence. Ie, a mouse enters the snakes sensory field (the antecedent), the snake strikes (the behaviour) and catches the mouse (the consequence). In this case, the snake has used instinct, however, it would also be learning and refining it's technique, becoming more efficient with every practice and that becoming more efficient is learning, or training in other words. This sequence of events is called operant conditioning in psychological lingo, more specifically operant conditioning using positive reinforcement. Learning can also happen using negative reinforcement, in which there is still an antecedent, behaviour and consequence, but this time, the consequence is avoidance or removal of an unwanted stimulus. Someone earlier in this thread gave an example of this when they described that their snake moved to the area where it would not be picked up from when it saw someone approaching (I know that's not the exact details, but I hope it's close enough). In this case, the antecedent is the approaching individual, the behaviour is to move to the "safe" spot and the consequence is being left alone. In both these cases, the behaviour is considered reinforcing, meaning that the snake is likely to utilise the same behaviour when presented with the same stimulus simply because it worked. Now I'm not saying snakes are the most intelligent creatures on the planet, but it has been shown that you do not need to be overly intelligent to respond this kind of basic learning or training. There is solid evidence (and you're going to have to forgive me for not knowing the reference of the top of my head) that tube worms were trained using a beam of light as a cue, to come out of there tube and receive a treat. In no way is it instinctive for a tube worm to see light then leave their tube assuming that they would find food, but they were trained none the less, in a relatively quick time, that the light beam stimulus (antecedent) meant that if they came out of their tube (behaviour), they would be rewarded with a tid bit of food (consequence). This was easily trained in that the stimulus could be presented many times in succession and the tube worm "rewarded" for it's behaviour of coming out of its tube. The trouble with using this theory for training snakes is that one of the easiest ways to train any animal is to use a primary reinforcer, something that an animal finds instinctively motivating, to reinforce a behaviour after a cue. In most cases this is done most simply using food, as with the tube worms. Using this method, we are a disadvantaged in formally training a snake in that best case scenario, they eat once a week, which would mean they've likely forgot what any overly novel action they took that resulted in the food last time. Consequently, we won't be seeing snakes doing circus tricks anytime soon, however, theoretically they would be learning to respond their environment and those who handle them constantly, like many animals more and less intelligent than they. And while I'm still ranting, as someone pointed out, there are monitors that have been shown to respond to novel cues in order to gain a food item. I believe that monitors, dragons and skinks would be much easier to train, not based on intelligence, but rather that they could simply be rewarded with small food items more regularly, giving them a chance to practice and perfect more novel behaviours.