system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does Horseradish only burn your nose?

Same goes for mustard I suppose. Why does it not affect my mouth as much, but makes me feel like I'm breathing fire out my nose?
gpt
The heat in horseradish comes from a highly reactive chemical ( isothiocyanate ). The chemical stays pretty stable as long as it isn't introduced to oxygen. When you chew on horseradish, you crush the plant (saliva also aids in breaking it down) and release this chemical which immediately reacts with the oxygen in the air and THAT is the moment it becomes hot. Now you breath even a little and its in your sinuses and burning ensues. If you breath a lot its in your lungs and you'll get a different crazy ass sensation when that happens.

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Mustard is similar, but different. It's water that kicks off the reaction there, but the chemical shares a commonality with mustard (the are both  phenolic  compounds) but that's probably far enough for ELI5.

Oh, fun fact, if you put vinegar in Horseradish very early after you grate or crush it, it will stabilize the reaction and keep it more mild.