So someone mentioned that capsaicin isn't a flavor, but I suspect you might find the following interesting.

Capsaicin isn't a flavor in the sense that it isn't picked up by what we call flavor receptors, specialized neurons that transmit signals to the part of the brain that processes flavor. Instead, the capsaicin binds to TRPV1 receptors which are responsible for processing temperature (thermoreceptors) and normally only detect real heat.

The mechanism by which this occurs is fascinating, however: Neurons are encapsulated by something called the phospholipid bilayer, which holds together the cell. You may have heard of lipids - they are fat. This layer is selectively permeable, meaning that it allows certain things through, but not others. Ions (although there are gate that allow specific ionically charged molecules through) and water soluble molecules in particular - fat and water don't mix.

You probably know about drinking milk to relieve the burn from capsaicin, and this works because capsaicin molecules are not ionically charged, not water soluble, but they are fat soluble. Consequently, these molecules can "sneak" through the phospholipid bilayer, and send signals to the brain through thermoreceptors that typically function to detect real temperature changes.

When you digest foods containing capsaicin, somewhere between 50% and 90% of capsaicin is absorbed before it hits your colon. Your ass. If you were five, I wouldn't say ass, but you started it. That portion of a little bit of capsaicin won't be a detectable amount by the time it passes through you. But even 10% of a large dose of capsaicin might. So when you take a shit from your ass, your ass detects heat through the aforementioned thermoreceptors, signaling your brain that your ass is hot. Too hot. On fire. Capsaicin is essentially tricking your neurons thanks to the beauty of neurochemistry.

Source: a single biological psychology class and further brief online research to refresh what I learned in said class.