If you watched Michael Phelps snag his 19th gold medal in Rio Sunday, you might also have noticed he had several red dots on his upper body like "a Gatorade bottle had given him a hickey," per Fox Sports. That's essentially what happened, but instead of a Gatorade bottle, glass cups did the damage. Phelps is among those to take part in the ancient Chinese medicinal practice known as "cupping therapy," featured in his own Under Armour ad. (See a demonstration here.) Traditionally, a flammable material is lit inside a series of glass cups which are then placed on a person's body, creating a vacuum-effect as they cool, adhering to the skin. An air pump can also be used to create suction.
Some believe the process stretches tight muscles and expands blood vessels, though there’s not much science to back that up, per the New York Times. "It kind of feels like someone is just pinching your back for five minutes … but the release is incredibly worth the five minutes of pain," swimmer Kayla Hutsell told Swimming World last year. The downside is that bruises remain when the cups are removed. An Instagram photo shows Phelps has been cupping for some time. The tell-tale marks have also been spotted on other Team USA athletes like gymnast Alex Naddour, who tells USA Today he bought a $15 cupping kit on Amazon. Jennifer Aniston and Gwyneth Paltrow are also fans, per the Guardian. (Read more 2016 Olympics stories.)