A bright, beautiful young mind asked the question, "does Earth's mass decrease when we burn fossil fuels? And if it does, is it measurable? Do we know how much of the Earth’s mass has been lost so far?" 
Well, Melaina, the Earth’s mass does decrease when fossil fuels are burnt. But not in the sense you were probably imagining, and only to a very, very small degree. Hear more about the geeky science of fossil fuels on this episode.