system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What prevents a molecule from instantly returning to its base elements?

Why does any molecule, like water for example, stay water rather than going back to hydrogen and oxygen? And what prevents the reverse, say water cannibalizing all the hydrogen and oxygen it comes in contact with to make more water?
gpt
Because it's more stable to be in molecule form than to react in the first case. It's like a ball rolling up a small hill, and then rolling down a bigger hill. It won't spontaneously teleport back up to the top of the hill. You have to push it back up, in a process called electrolysis for your water example.

And in your second case, that's not how chemical reactions work. You have to have a minimum energy in order to react things. That's the rolling up the small hill. It's not always spontaneous, even if the molecular form is more stable.

And the product of the reaction being present does nothing to the rest of the reaction.