Why my touchscreen doesn't respond to a pencil? I noticed my phone responds to my finger, but not to a plain graphite pencil. 
Why is that, what is its physical reason?
How are styli able to approximate the human touch? What kind of conductive materials are being used there?
 A: Capacitive touch screens work by measuring capacitance at a number of locations on the screen, then interpolating between them to get a precise position.  The simplest form of capacitor is two parallel conductive plates on opposite sides of an insulator, with the capacitance being proportional to the area of intersection of the two plates and the dielectric constant of the insulator, and inversely proportional to the distance between the plates.  In the case of a touch screen, one plate is a transparent conductor on the back side of the glass, and the other is the user's fingertip.  A pencil does not work well for this, even if made of metal, because its tip has a very small area.  Styli made for this purpose usually use a tip that is at least 6 mm in diameter, which deforms when you press it against the screen, enlarging the contact area.  Depending on the exact measurement technique being used (some touch screens measure mutual capacitance between two conductors on the screen, and others effectively measure capacitance to ground), your stylus may or may not need to be in contact with a human body to work.
