There was a story I once read about a musician who was nicked for speeding in his vintage car which had no working speedometer. In court he explained that he could tell what speed he was doing by the sound of the gearbox, and at 30 mph it whined at A flat.
Carbohydrates provide the bulk of the fuel for our bodies. They are “burned” with oxygen (releasing heat) and turn into water and Carbon Dioxide.
Happened to me last time I was in my doctor’s office for a checkup and the nurse had trouble finding my pulse.
No trouble believing that, at all. Fun ‘n’ E-Z experiment you can try at home: next time you’re humming a tune in the shower, after you towel off, grab yer blow-dryer and see if what you’re humming can match the pitch of yer BD. If you reach the pitch, (women will have a better time than men as their voices are higher) the humming will sound twice as loud in your head, and if you can waver a little above or below the pitch, you’ll get nifty vibes in your head. I freaked out Mrs. Burpo with that last week (She has a Master’s in music, and didn’t know some things have their own pitch–the things they teach in college, nowadays! 
) Big laffs!
So… Why aren 't “cold blooded” animals (snakes come to mind) able to heat themselves?
They do, but for whatever reason they have not evolved the same mechanisms that ‘warm-blooded’ animals have for regulating and keeping that heat. They just lose it to the environment.
They do: if they are very active their bodies heat up just like a mammal’s body would.
What they don’t is a thermostat. When a mammal gets cold, it speeds up its metabolic processes to generate more heat to stay at the same temperature (and also does a bunch of other things to retain generated heat). A snake just lets itself get cold.
Also, shivering. Shivering is just your body making otherwise useless muscle movements for the purpose of generating waste heat.
Most “warm-blooded” creatures also have insulation like fur or feathers, which most “cold-blooded” creatures lack.
As well as sweat glands, etc. to shed excess heat, which is equally crucial. In fact, that’s what makes us naked, sweaty humans the best endurance runners on the planet. 
Actually, we do too have a specific “heat organ” – lots of them, in fact, strategically distributed throughout the body. They are called [del]midi-chlorians[/del] mitochondria (singular: mitochondrion), and most cells contain or or many of them.
Not to get too metaphysical, but when a snake, eg, gets “cold,” does it feel cold, ie, uncomfortable, … Hell, any sensation like those that snakes do have?
Or does it–usually? always?–slow down its metabolism (ie, “always” means that’s the way it’s built) so it all evens out in the end?
I always sort of wondered this since I was a kid. Thanks to this thread.
A word I’ve always loved and don’t get to use much: when the temp drops, “cold-blooded” animals go into torpor.
Not sure, but I don’t believe “cold-blooded” animals have any kind of fat deposits to draw on.
Cold-blooded animals are exothermic;i.e., they get their heat from the environment. Warm-blooded animals are endothermic; i.e., they produce their own heat. Endothermics need a lot of food to produce heat. They have to eat at least once daily. Exothermics, not so much, and their need for food is not daily. For instance, an alligator can go for weeks without eating. That is why you see turtles and ducks and other water life next to a gator. They seem to know when it is not hungry. In the cold mornings, exothermics are not active. They wait for the heat of the day to warm them up sufficiently.
IIRC some lizards have large fat reserves in their tail to buffer against hungry periods. I’d suspect other lizards do too.