Are reptiles ectothermic or endothermic?
Reptiles do not have the ability to maintain a constant internal temperature by physiological processes; their body temperature is dependent on the ambient temperature. This is why they bathe in the sun; they must alter their behaviour in order to maintain a relatively stable internal temperature, despite changes in the ambient temperature.
Endotherms (like us) are able to create their own heat through metabolic reactions (these use energy to create heat). As reptiles are unable to do this, they have to maintain their body temperature another way, mostly by changing behaviour.