The words "carbon footprint" are often seen in the news. They are used with the word sustainability, which means that we can keep doing something without making something bad happen. That is, if there is too much of a carbon footprint, then there is no sustainability. People and businesses are often asked to think about how much carbon they are putting into the air, and how this changes the environment and maybe causes global warming. This is often said about traveling by airplane or by car, because burning fuel makes carbon dioxide gas.