I should probably be embarassed to write this, having a degree in Electrical Engineering and all that. Doesn't matter, you should never be afraid of your own stupidity, I think. The best way to learn is from your own ignorance.

Anyway, a bit of a background in physics helps in this matter, but it's not absolutely essential. So here it goes.

Everything that has a temperature above absolute zero (i.e. 0 Kelvins) radiates energy. The amount of energy, and the wavelength of this energy, depends on the temperature. If you went to high school and took some physics classes, you'll probably remember this. If not, think of science fiction and spy movies, where someone peers through infrared goggles and can see at night. Or something like that.
Now, it's not necessarily infrared radiation that is being radiated by objects of a certain temperature. Take a piece of iron. If you heat it up, it becomes red ("red hot"). Heat it up more, and it becomes white ("white hot"). Take a light bulb. The whole reason the thing functions at all, is that the filament inside it (made of tungsten, usually), is heated to such a temperature that it starts radiating not just infrared radiation, but also radiation in the visible spectrum (light, so to speak). The big part of the energy radiated is still infrared, by the way. Touch a lit bulb and you'll know what I mean. The thing gets hot. The relationship between temperature and distribution of wavelengths of the radiation has been figured out a long time ago by some very smart folks (a guy named Boltzmann, to name one).
So that's all great. Nothing new here. But why does stuff emit radiation?

Slight diversion here. In the 19th century, a very intelligent man named James Maxwell figured out the relationships between electric and magnetic fields, and wrote them down in a few rather elegant equations. I'm not going to bore you with those. Most importantly for this discussion though, is the consequence that a changing electric field causes a changing magnetic field, which causes a changing electric field, and so on. This causes electromagnetic radiation. This is rather important, since visible light is electromagnetic radiation, and what would we do without it? Bump our heads all the time, I guess. So, the way to generate electromagnetic radiation is to have a changing electric field (or a changing magnetic field, but let's not overcomplicate things).

Another slight diversion (don't worry, it'll all come together at the end). What exactly is temperature? I don't want to go into all the details (I would have to look them up), but one of the things is that the atoms in an object tend to vibrate more as their temperature goes up. So, all atoms in an object vibrate (i.e. move) to a certain extent. The hotter the thing gets, the more vibration, so the more movement.

Aha! Right! Since atoms are made up of electrical charges (positive nuclei surrounded by clouds of negative electrons), a vibrating atom will cause a changing electric field. Which causes a changing magnetic field, which causes.... Which causes electromagnetic radiation. Which is nothing more than a fancy word for "light", be it infrared or otherwise.

Rather obvious, huh? Once you realize it, that is. Call me stupid, but it took me a long time to fully realize this.

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