I heard that bananas are radioactive. If they are radioactive, then they radiate energy. How many bananas would you need to power a house?

Bananas are radioactive because they contain potassium, some of which is the radioactive isotope potassium-40. The factoid about banana radioactivity was popularized by nuclear engineers trying to reassure people[1] that small doses of radiation are normal and not necessarily dangerous. Of course, this kind of thing can backfire.
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RSK: And now you have the rest of the story!