Snakes must lie out in the sun, or on warm surfaces to raise their core body temperature because they are ectothermic. This is known as basking. Raising core temperature is critical to maintain normal functioning of their systems. For example in 2010 Florida went through a long cold spell which plunged temperatures well below normal. It was found that snakes who hadn't been able to warm themselves up via basking were often unable to properly digest their food.