The ball in a ballpoint pen provides a way to dispense fast-drying ink and keep the ink from drying out while it's still in the pen. The ball is like a cap to keep air from getting in to dry out the pen's ink, but at the same time letting the right amount of ink out of the pen and onto the paper. The ball sits tightly in a socket between the ink and the paper, but it's still loose enough to be able to roll as you write. The back of the ball picks up the ink that gravity forces down toward the paper, and as you write, the ball turns and transfers the ink from its back to its front. If the ball isn't turning, the ink inside the pen is sealed tight.
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