Ballpoint pen artwork  Since their invention and subsequent proliferation in the mid-20th century, ballpoint pens have proven to be a versatile art medium for professional artists as well as amateur doodlers. Ballpoint pen artwork created over the years have been favorably compared to art created using traditional art mediums. Low cost, availability, and portability are cited by practitioners as qualities which make this common writing tool a convenient, alternative art supply. Ballpoint pen enthusiasts find the pens particularly handy for quick sketch work. Some artists use them within mixed-media works, while others use them solely as their medium-of-choice. The medium is not without limitations; color availability and sensitivity of ink to light are among concerns of ballpoint pen artists. The internet now provides a broad forum for artists to promote their own ballpoint creations, and since its inception ballpoint pen art websites have flourished, showcasing the artwork and offering information of the usage of ballpoint pens as an art medium. Some of the most famous artists of the 20th century have utilized ballpoint pens to some extent during their careers. Andy Warhol and Alberto Giacometti both used ballpoints within their artwork in the 1950s. Cy Twombly exhibited small ballpoint drawings in the 1970s. Ladislao Biro himself utilized his own invention creatively; a 2005 mechanical engineering exhibition in Argentina, focussing on the invention of the ballpoint pen, included in its brochure a ballpoint pen drawing titled "Waiting" credited to Biro. The popular Spirograph, mass-marketed in America during the advent of 1960s psychedelic culture, included colored ballpoints (black, blue, red, green) as part of its boxed set. The holes positioned on a Spirograph's "gears" were reportedly sized to accommodate tips of the fine-point pens provided. Artists now professionally employing ballpoint pens often cite classroom boredom as a factor allowing them to explore the writing instrument's creative applications. A mainstay of school-supply lists, students use their ballpoints to doodle onto folders, desks, and blue jeans—even onto each other. Artistic aspirations aside, the average person may pick up a pen during lengthy telephone calls, consciously-or-not scribbling Hitler mustaches and black-eyes onto magazine photos of politicians or models; artist Jean Dubuffet has admitted to having realized the potential of ballpoint pens in this manner. Ballpoint artist Lennie Mace has stated that he learned the basics of anatomy and perspective in his youth by tracing over newspaper photos in ballpoint pen, a practice which evolved into his "Media Graffiti" embellishments of print-ads.