It's called New Mood Swing Emotionally Activated Lip Gloss by Too Faced, and according to the product description, it changes from pale pink to crimson red when the wearer becomes aroused.
"Are you in love jealous, angry, happy or hot and bothered? Your lips will tell the tale and allow you to express yourself like never before," reads the product description on ASOS.com, a beauty shopping site.
It alleges to do this by reacting with a woman's body chemistry.
The product doesn't explain the science behind mood lipstick any further than that. The '70s phenomenon mood rings, however, simply function as thermometers, changing colour with the body's temperature.
Reactions to the gloss have been mixed.
An anonymous review from the Sun in the UK says it works perfectly, finding her lips went pale when looking at images of men she wasn't attracted to, and dark red when she looked at celebrity crushes, like True Blood's Stephen Moyer.
"After just seconds of puckering up to him on my screen, I looked like I'd necked nine pints of O Negative," reads the review.
Others are less than impressed.
Adam Frucci, a writer for the science and technology blog Gizmodo, expressed doubts about the product's legitimacy.
"And even if it does work, I'm not sure what the benefit is. I mean, do you ladies out there really want to show your cards that obviously," wrote Frucci.
Kate Harding, a writer for Salon.com, worries the lipstick might give men the wrong idea.
"I do worry that a marketing strategy like this (tongue-in-cheek though it may be) reinforces the pernicious myth that figuring out whether a woman wants to have sex is some daunting puzzle far beyond the intellectual and emotional capabilities of the average Joe," she wrote "It's actually pretty simple. If you have any doubt, ask."
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