A fountain pen is a pen that contains a reservoir of water-base liquid ink. The ink is fed to the nib through a feed via a combination of gravity and capillary action. Filling the fountain pen reservoir with ink may involve replacing a disposable ink cartridge, filling the pen with an eyedropper, or operating an internal mechanism which sucks ink into the reservoir from a bottle through the nib. These mechanisms are typically pistons or rubber sacs. Pens that accept cartridges are often made to suck ink through the nib by replacing the cartridge with a syringe-like device called a converter.