Scientists have discovered why buttercups glow yellow under people's chins - and it has nothing to do with liking butter.
Researchers found that the flower's unique anatomical structure - used to attract pollinating insects - contributes to the popular children's trick.
Experts in physics and plant scientists from Cambridge University worked together to solve a problem that has perplexed generations of researchers.
Dr Beverley Glover, from the department of plant sciences at Cambridge University, said: "This phenomenon has intrigued scientists and laymen alike for centuries.
"Our research provides exciting insight into not only a children's game but also into the lengths to which flowers will go to attract pollinators."
The researchers discovered that the buttercup petal's bright and glossy appearance is the result of the interplay between its different layers.
The strong yellow reflection responsible for the chin illumination is mainly due to the epidermal layer of the petal that reflects yellow light with an intensity that is comparable to glass. The researchers also found that the buttercup reflects a significant amount of UV light.
As many pollinators, including bees, have eyes sensitive in the UV region, this provides insight into how the buttercup uses its unique appearance to attract insects. Scientists had previously shown that the reflected colour is yellow due to the absorption of the colours in the blue-green region of the spectrum by the pigment in the petals.
Dr Silvia Vignolini, from the university's department of physics, said: "Although many different factors, such as scent and temperature, influence the relationships between pollinators and flowers, the visual appearance of flowers is one of the most important factors in this communication.
"Flowers develop brilliant colour, or additional cues, such as glossiness - in the case of the buttercup - that contribute to make the optical response of the flower unique. Moreover, the glossiness might also mimic the presence of nectar droplets on the petals, making them that much more attractive."