A ballpoint pen is a writing instrument that dispenses ink via a metallic sphere at the pen's tip. There are two main types of ballpoint pens: disposable and refillable. Disposable pens contain a fixed plastic ink cartridge and are discarded when the ink is used up. Refillable pens have a removable ink cartridge that can be replaced when empty. Disposable pens are made from plastic while refillable pens are made from either metal or plastic. Some ballpoint pens have a retractable tip that is attached to a spring, which can be retracted or deployed by a button at the top of the pen. Ballpoint pens are cheap and durable and are the writing instrument of choice in modern society.