They say exercise gives you a natural high, but a gym in San Francisco is making doubly sure. Power Plant Fitness, due to open in the city this autumn, touts itself as the world's first cannabis-friendly gym. The venture was dreamed up by Ricky Williams, a believer in the athletic benefits of weed consumption. Some people think the green stuff aids focus and increases lung capacity. Power Plant will allow gym-goers to consume weed while working out, and to buy a range of cannabis-based supplements and snacks. Williams knows his stuff when it comes to fitness- he's a retired NFL Pro-Bowler. Also when it comes to weed ñ he left the sport after repeatedly testing positive for marijuana. The only downside is, if it's weight loss you're after, this may not be the place for you. One word: munchies.