Bananas are naturally slightly radioactive due to their potassium content.
Why it is true
They contain potassium-40, an unstable isotope that decays by emitting radiation. The dose is so small that you would have to eat millions of bananas at once to suffer any negative health effects.
Source: EPA
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.