Long story, short: someone was assaulted with a hot cup of coffee yesterday, and I wound up discussing the impacts of caffeine on the skin.
Caffeine CAN be absorbed through the skin (which was a running joke in the Sylvia comic strip by Nicole Hollander, in which the protagonist started developing elaborate wardrobes for her cats after using caffeinated soap, and the cats begged her to stop), and is a fashionable item in cosmetics (it constricts blood vessels, and so can reduce some types of swelling).
But somehow, I wound up reading that caffeine is also a highly effective pesticide against invasive species of frogs in Hawaii. This makes some sense to me, based on my limited understanding of amphibians and their sensitive, not-especially-protective skin.
Anyway, here’s the most scienc-y summary of what I read. It’s called, “Hawaii’s Hated Frogs:
Tiny invaders raise a big ruckus.” A frog mating call loud enough to damage human hearing is a hazard!
Janet Raloff Puerto Rico’s beloved mascot is a miniature tree frog named for its distinctive call: ko-KEE. All night long, choirs of love-starved males serenade would-be mates, who respond with quiet guttural chuckles. “To me, it’s pleasant-just like birds singing,” says Bryan Brunner, a University of Puerto Rico plant breeder in Mayaguez.