Why do buttercups light up your chin?

It’s a happy memory that many of us have from childhood: picking a buttercup, then running to a friend or family member and holding it under their chin. If it lights their chin up yellow, that means they love butter, according to the game.

But it’s not really about the butter: whether the person loves it or not, light reflecting from a buttercup bathes their chin in a yellow glow – so why is that?

A paper out this month claims to have new insights about the flower, which has a glossiness that not only gives kids a giggle but could also help attract pollinators to help it reproduce.

Scientists have been puzzling over the glossiness of Ranunculus repens since as far back as 1883, but the new study used electron microscopy and spectroscopy to probe the anatomy and optical properties of the flower’s petals.

The researchers saw two flat surfaces in the petal from which light can be reflected – the smooth surface of the outermost “epidermal” cells and an air-containing layer underneath – making the flowers good reflectors.

The researchers, from the University of Cambridge, also found that the buttercup reflects a substantial amount of UV light, which might be a lure for insects.

"The directionality of light reflection creates a dynamic intensity pattern that could potentially be recognised by pollinators," they write in the Journal of the Royal Society Interface.

“The reflected light would appear as a flash, which could initially capture their attention.”

Just don’t tell the kids and ruin the butter story.