Get up close and personal with a perky buttercup and you'll notice that the yellow flower's sunny petals have a distinctive sheen. (Maybe that's why they're the flower little kids pluck and hold under each other's chins, looking for a special yellow reflection to find out "if you like butter.")
This special reflective quality is unique to the buttercup (known officially as the Ranunculus genus) and researchers discovered that it's created by a special combination of pigments and structure create a thin film as the petal's outer layer.
"A very flat, pigment-filled upper epidermis acts as a thin film reflector yielding the gloss, and additionally serves as a filter for light backscattered by the strongly scattering starch and mesophyll layers, which yields the matte yellow color," the researchers write in their study, published in the Journal of the Royal Society Interface.
That shininess may serve several purposes, study author Caper J. van der Kooi of the University of Lausanne in Switzerland tells Live Science.
When the flower's petals are spread wide on sunny days, their glossiness might act like a mirror reflecting a sunbeam, causing the flowers to stand out to pollinators.
On less sunny days, the sheen can help keep them warm. As Live Science explains:
"The buttercups also might be acting as their own space heaters. On overcast days, the researchers observed that buttercups close into a cup-like shape but also tilt their blossoms to follow what sunlight they can catch. (This is called heliotrophy.) The effect is to reflect light waves not toward the sky, but internally, toward the plant's reproductive organs."
This maneuver warms the center of the flower, which may help with their pollen and seed production. Another plus? Some insect pollinators just prefer warmer flowers, making buttercups that much more attractive.