Fiber is a thread-like strand that makes up many materials. Cotton fibers twist together to form yarn for your t-shirt. Wood fibers pressed together create paper. Muscle fibers bundle to give your body strength. Even the cables that carry internet signals to your home are called fiber-optic cables because they use tiny glass fibers to transmit information as pulses of light.

The word also describes a type of substance in food that your body can't digest but needs anyway. Fruits, vegetables, and whole grains contain fiber that helps your digestive system work properly, like a broom sweeping through your intestines. When you eat an apple, the fiber fills you up and keeps things moving smoothly.

You might also hear fiber used to describe someone's character or nature, as in “every fiber of her being wanted to win the race.” This means her entire self, down to the smallest part, felt that desire.

Scientists and engineers care deeply about fibers because their properties determine what we can build. Carbon fiber is incredibly strong but lightweight, making it perfect for race cars and bicycles. Natural fibers like wool keep you warm. Synthetic fibers like nylon resist water and last for years. Understanding fibers means understanding the basic building blocks of countless things around you.