Researchers from Cambridge University have found that the buttercup’s unique anatomical structure is why they glow yellow under people’s chins.
(The plant has evolved an epidermal layer on its petals that reflects a significant amount of UV light with an intensity that is comparable to glass, the plant has developed this to attract pollinating insects, but of course it is also used to tell if you like butter or not. In the UK a children’s game involves placing a buttercup under your chin, and if your chin glows yellow, it proves you like butter.)