You've heard of 3D printing from newspaper and TV programs. It can "print" in plastic, metal, nylon, and over a hundred other materials. 3D printing technology has been around since the 1980s, it was not widely used until the early 2010s. The first working 3D printer was created in 1984 by Chuck Hull. 3D printers use many different types of technologies, but they all share one thing in common: they create a three-dimensional  object by building it layer , by continuing layer, until the whole object is completed. Imagine a baker lay down each layer one at a time until the whole cake is formed. 3D printing is somewhat similar, but just a bit stricter. How can 3D printers work so well? The most important thing-a program, a 3D printer also needs to have instructions for what to print from a computer. It needs a file as well. The file which is a Computer Aided Design (CAD) paper is created with the use of a 3D modeling program. The program creates a paper that is sent to the 3D printer. Along the way, software pieces the design into hundreds, or more likely thousands of layers. The 3D printer builds up one layer at a time, from the bottom to the top, until the object is done. 3D printings can be used for making many kinds of things such as machine parts, daily life objects, even human organs  using a person's own cells. Nowadays, Marc. Dillon founds a project which deals with many world famous paintings like Leonardo da Vinci's Mona Lisa, Van Gogh's Sunflower with 3D technology. This project helps the blind people enjoy the fantastic art through touching. In order to provide a good user experience, these 3D paintings have special instruments which can tell people what they touch. According to the passage,   _  can help the blind know what they touch.
CAD paper Machine parts. The world famous paintings Special instruments