A ballpoint pen is a writing instrument that uses a tiny rotating ball at its tip to spread ink onto paper. The ball, usually less than a millimeter in diameter, sits in a socket and rolls as you write, picking up thick, oil-based ink from a cartridge inside the pen and depositing it smoothly on the page.

Before the modern ballpoint pen was developed in the 20th century, many people wrote with fountain pens that used liquid ink and could easily leak or smudge. The ballpoint changed everyday writing because it leaked less, dried quickly, and worked reliably at many angles. This made it perfect for everything from everyday note-taking to use in challenging conditions, such as on airplanes.

The invention succeeded because of that clever ball design: as the ball rotates, it acts as a seal to help keep the ink from drying out while also spreading it evenly. When you see “ballpoint” in a store, it distinguishes these pens from gel pens, felt-tip markers, or other writing tools. Today, ballpoints are so common and inexpensive that billions are manufactured every year, though the simple mechanism inside each one remains very similar to the early designs.