Britney Spears is warning everyone not to "try this at home." The 39-year-old pop princess, still fronting media outlets for newly-released "Matches" collab with Backstreet Boys, today updated her Instagram with an unusual snap. Britney, followed by 27 million, posted a sizzling outdoor workout shot as she ditched the yoga for some fresh air and cardio. The "Toxic" singer was confident with hands on hips in the sunny image, but the caption told a different story. Check it out below.