system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the difference between reptiles and mammals that allows one to be warm blooded while the other can't create its own heat?

As far as I know all the major organs are roughly the same between cold and warm blooded creatures - lungs, hearts, brains. Do mammals have an extra organ that lets us produce our own heat?
gpt
some cold blooded animals do generate their own heat by muscle action, but they do not maintain a specific, stable body temperature. At rest, cold blooded animals slow down heartrate, respiration, digestion,  etc

Active predatory fish like tuna have powerful muscles that make a lot of heat, and help the fish to move fast and swim hard to catch it's prey.

A snake at rest will stay cool, warming up in the sun when it wants to, but they can turn up the temperature metabolically when they are digesting a big meal.

Mammals basically "keep the heat on" all the time with a high metabolic rate *even at rest*, so just the action of digesting, breathing, growing, etc uses energy and produces heat as a byproduct of that energy use.