During summer most people will eat copious amounts of pineapple. Usually this is accompanied by a weird raw feeling in your mouth and then a stinging sensation.
Well it turns out this is caused by a mixture of enzymes called bromelain. When bromelain comes into contact with sensitive skin, like that in the mouth, it starts to break down the proteins, causing the tissue around your mouth to become inflamed and rather sore.
Special shout out to iflscience for bringing this to attention.