Ballpoint pen...

This drawing shown here is a piece by Juan Francisco Casas created using only a blue ballpoint pen. Pen ink in art is considered to be a liquid media. Although it could be perceived as a photograph, if you look closely you can see the massive amounts of cross-hatching used to create the negative space in the picture. The pen ink is also used for shadowing on the photographer's body to create the illusion of implied light as well.

-Alfonso Robles