Answer: Bananas

In 2008, researchers working on a collaboration between the University of Innsbruck and Columbia University discovered a novel way to test bananas for ripeness: using a black light.

The yellow appearance of bananas under normal light is mainly due to natural pigments (carotenoids). But as bananas ripen, the chlorophyll in them breaks down and will cause them to fluoresce brightly (with a blue glow) under a black light. What’s most interesting about the discovery is that it appears to be unique to bananas, other plants don’t exhibit the same fluorescing process during the breakdown of their chlorophyll.