Portable Piston Fill Fountain Pocket Brush Pen BH-A20 The top of brush handle is a piston knob that controls the piston-filler and adjust the flow while using. The exterior of these brush pens is like a regular fountain pen with a metal cap. This pocket version piston brush pens are suitable to fill with ink, liquid watercolor or just water. The brush pen is really like an actual natural hair brush. No more worries of being dried out or too much inks flowing because of the piston. It doesn't need any expensive special ink. It is a portable brush that you can draw outside too.