What do reptiles do to warm up?

Reptiles are ectothermic animals (poikilotherms). They do not produce heat to warm their own bodies. They rely on the heat from their environment to warm up. This is why reptiles are seen basking in the sun during the daytime.

If a lizard in the desert feels the need to warm up for its metabolism to perform more efficiently, it will sit on a rock in the sun. However, to avoid overheating, it goes behind a rock in the shade.