 Yes, cement. We use it to glue together just about everything we build. But the production of cement worldwide actually emits almost as much carbon as cars do. The majority of that carbon comes from one of the main ingredients in cement, which is limestone. To make cement, you basically cook limestone at high temperatures. But limestone is actually a carbon sink. It contains a lot of carbon. And when you heat it up to make cement, that carbon is released into the atmosphere.