What is the most common user interface in the world? [closed]

Please explain:

• (1) What leads you to believe the interface is the most commonly used, and
• (2) Why do you believe that it is an interface.

UPDATE: Leonardo Herrera's answer "The button, and its cousin hyperlink" had been selected as the answer, but a number of users took issue with this answer, so I'm opening the question back up, and have attempted to address the issues raised by adding clarifications to the question itself.