Summarize this article:

There are several different types of touch screens and your homemade stylus may not work with all of them.  iPhones, iPads, Androids, Kindles, and many other smartphones and tablets have a capacitive touch screen, which needs an electrical conductor (like the human body) to register where contact is made. Nintendo DS, The 3Ds, the Nook, and some other phones and e-readers use resistive or infrared touch screens, which only require pressure to register contact. You can use just about anything as a makeshift stylus--just be careful not to scratch the screen.
Find out which type of touch screen your device uses.