Simplified: CO2 in soda has an effect on acidity, which gives a pain/burn that normally you don’t detect that well. Capsaicin or other spices are hot, so they both give a pain/burn, so imagine adding 1 + 1. We know 1 + 1 + 2 so we can prove 1 pain + 1 other pain = 2 pains, or in other words, the pain gets stronger.
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