Question

When a snake gets too cold, it will move to a warm rock. It sits on the warm rock to feel heat from the Sun and increase its body temperature.

Answer 1

Reptiles use the environment to regulate their warmth. This strategy helps them conserve energy in the cold and rapidly warm up their body temperature in the heat.
If a snake feels like it needs to warm up for its metabolism to work better, it goes and sits on a rock in the sun. If it gets too warm, it goes and sits under a rock in the shade.