A rollerball pen is a pen that utilizes ball point writing mechanisms using water-based liquid or gel ink. The more common ballpoint pens use oil-based viscous inks. Roller ball pens have more distinct writing qualities due to the less viscous inks that saturate more widely and more deeply on paper. The writing point is a tiny ball, about 0.5 to 0.7 mm in diameter. Due to the water-based ink a cap is needed to prevent the ink from drying out.