system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't light "overfill" closed containers like gas and liquids do?

I feel like the easy answer to this is "Because light has no mass." But where does the spent energy go?

If you create an enclosed container and keep adding water to it, eventually the container will overfill and the container will burst. The same can be said of gas.

What makes light so different? 

If you create a similar space and put a lightbulb in it, that lightbulb is providing a constant "flow" of light in the same sense that a stream of water provides a flowing source of water, but the stream of water pools up at the bottom of the container. What is the equivalent of that pool, for light?

By comparison, if you provide the constant source of gas, the gas expands throughout the container and is compressed because there's nowhere else for it to go. Why can light escape this when gas cannot?

EDIT: Wording error.
gpt
This is because you can't have two atoms in the same spot. Atoms occupy physical space, they have volume. Photons, though, don't. They don't have mass nor volume and don't occupy space. Photons are local excitations of a field, but you can think of them as a wave in a pool. Can a pool have too many waves? When two waves are at the same spot, they interact and interfere, but both are still there.

So if you had a box of perfect mirrors and a source of light inside it, you would have it bouncing around eternally, with more and more and more photons inside but never having too much.

However, in our world, there are no perfect mirrors. Whenever light reflects out of something, a portion of it is reflecting but another portion os absorbed and turned into heat. So if you had a real box of mirrors with a source of light inside, you would notice it becoming ever so slightly hotter, as part of that light is absorbed and heats up the box.