Your tears have a secret topping: a thin layer of oil. It comes from tiny glands hidden inside your eyelids.
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Inside each eyelid is a row of long, skinny meibomian glands, standing side by side. Their tiny openings sit along the edge of your eyelid, just behind your eyelashes.
The cells inside slowly fill up with oil, like little balloons filling with butter. When a cell is full, it breaks apart and the oil flows out. New cells keep growing to take its place.
Each time you blink, your eyelids spread a thin film of oil over the watery tears.
Press Blink on the 3D model to see oil come out of the gland. Then try a staring contest and notice how your eyes start to feel dry. Blinking spreads the oil again!
Oil floats on water. That's why this oil can sit on top of your tears like a thin blanket.