Ok, so what's the deal with cold-blooded organisms?

Im not studying biology but in primary school i was always taught that cold-blooded creatures like reptiles and fish can't produce heat. Since im in engineering, i know that is just not possible. So what's the real reason reptiles like crocodiles, for example, can't move quickly if they are not properly heated from an outside source? Can't they just burn some ATP for the heat alone? wouldn't they heat up of they moved, taking advantage of the heat that exothermic chemical reactions release?