Cows are already the nation’s single largest source of methane, a greenhouse gas produced by oil extraction, decomposing trash, and the guts of grazing animals that are as much as 105 times more potent than carbon dioxide. A single cow farts and belches enough methane to match the carbon equivalent of the average car. So scientist and a group of interns decided to capture the methane by attaching a bag that hooks into their stomach to capture the excess.

https://www.motherjones.com/environment/2015/12/discovery-racing-extinction-methane-bags-timelapse/