You burn a piece of paper. After burning the paper, you find the mass of the remaining ash. The mass of the ash does NOT equal the mass of the paper you started with. How can you account for the different in mass? Where could it have gone?

When paper is burned oxygen from the air combines with carbon and hydrogen in the paper turning some of it into carbon dioxide and water vapor, which waft away with carbon particulates in the smoke. This, not surprisingly, leaves the solid ash leftover lighter than the original paper.