Pineapple contains an enzyme that digests the proteins in your mouth while you eat it.
Why it is true
Fresh pineapple contains bromelain, a mixture of enzymes that break down proteins. This is why the tongue can feel sore or tingly when eating fresh pineapple, as the enzyme is literally digesting the cells of the oral mucosa.
Source: Journal of Agricultural and Food Chemistry
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.