system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that spicy condiments like sriracha and brown mustard are at their full spiciness when you first open the bottle but, over time, they lose their punch?

Like, right now, I cracked open a bottle of spicy brown mustard and it's clearing out my nasal passages. But, three weeks from now, I'm not going to get that same burn. Is it me adjusting to the heat level or the condiment losing its burn?
gpt
capsaicin and sinigrin, the spicy components in chilis and mustards respectively, can oxidize when exposed to oxygen and lose some of their spice. 

edit: for a little more detail, capsaicin binds to very specific neuronal receptors in the body that are related to pain and heat. When the capsaicin molecule binds, it acts like a key opening a lock. The action of opening the lock causes your brain to experience it as heat or pain, because that is the specific type of lock (receptor) being opened (activated). When the capsaicin molecule is exposed to excess oxygen molecules for extended periods of time, its structure can change, thus the "key" may no longer fit into the "lock" and you experience less pain/heat.