A capillary is an extremely thin tube or channel, so narrow that liquids can move through it in surprising ways that seem to defy gravity.

In your body, capillaries are the tiniest blood vessels, so small that red blood cells must travel through them in single file, like students walking through a narrow hallway one at a time. These microscopic tubes connect your arteries (which carry blood away from your heart) to your veins (which carry blood back). Capillaries bring oxygen and nutrients right up against your cells and carry away waste, making them essential for keeping you alive even though each one is thinner than a human hair.

The word also describes any very thin tube where liquids can climb upward on their own through a process called capillary action. When you dip the corner of a paper towel in water and watch the water creep up the towel, that's capillary action at work. Plants use capillary action to help draw water from their roots up through their stems to their leaves. The liquid molecules are attracted to the walls of these tiny tubes and to each other, creating an upward pull that seems almost magical.

Scientists and engineers use glass capillary tubes in laboratories for precise measurements, since the tubes are so thin they can hold and move tiny amounts of liquid with accuracy.