Robert F. Kennedy Jr. is going viral on social media after he was caught on camera adding a mysterious blue liquid to his in-flight beverage, and internet sleuths believe they know what it is. The leading theory among X users is that the prospective health secretary was dosing his drink with methylene blue.

RFK Jr, a known vaccine skeptic and conspiracy theorist, was filmed administering several pipette-fuls of the Cobolt blue-colored liquid into his glass. The video emerged on Wednesday as the 72-year-old awaits Senate confirmation to be the nation’s top health official.