Pen

A pen is a writing instrument that uses liquid or gel ink to leave a visible mark on a surface, usually paper. A common ballpoint pen contains a small metal ball at the tip, held in a socket so that it can rotate freely. As the pen moves across the paper, the ball rolls and picks up ink from an internal reservoir, then transfers it onto the surface. The ink is designed to flow at a controlled rate and dry or set quickly enough to avoid excessive smudging.

The body of the pen protects the ink reservoir and provides a comfortable way to hold and guide the instrument. Different pens use different inks and writing mechanisms. Ballpoint pens generally use oil-based ink, while gel pens use a water-based gel containing pigments or dyes. Rollerball pens typically use a more fluid, water-based ink that produces a smoother line but may take longer to dry.

The shape and size of the pen's tip affect the width and appearance of the line. Some pens have retractable tips operated by a button, while others use a removable cap to protect the writing point from drying out or being damaged. Many pens also include a spring-loaded mechanism that controls the movement of a retractable tip. Although pens are simple everyday objects, their operation depends on several small design features.

The ink must remain stable inside the reservoir while still reaching the tip, and the writing point must release enough ink to form a clear line without leaking. Materials such as plastic, metal and rubber are commonly used for different parts of the instrument. Pens are widely used for handwriting, note-taking, drawing and marking, making them one of the most familiar writing tools in daily life.