Cement is a fine gray powder that, when mixed with water, forms a paste that hardens into a strong, rock-like material. When you add sand and gravel to cement and water, you create concrete, which is used to build sidewalks, buildings, bridges, and roads all over the world.

The hardening process is a chemical reaction that bonds materials together. Once cement sets, the bond is very hard to undo. This makes it incredibly useful: ancient Romans built structures with cement over 2,000 years ago that still stand today. Modern cement is one of the most widely used substances on Earth.

People often confuse cement and concrete, but they're different: cement is an ingredient in concrete, like flour is an ingredient in bread. You wouldn't eat plain flour, and you wouldn't use plain cement by itself to make a driveway.

The word also means to strengthen or make permanent. When a team wins the championship, that victory cements their reputation. When you practice a skill until it becomes automatic, you're cementing it in your memory. When two friends overcome a challenge together, the experience can cement their friendship.