A Piezo is an electronic device that can both be used to play tones and to detect tones. In this project, a Piezo captures a knock and sends a string "Knock!" back to the computer over the serial port. In order to see this text you can use the Arduino serial monitor.
Connect the piezo's red wire to Arduino's analog pin 0, the black wire to ground, and a resistor in the range of the Megaohm parallel to the piezo element. Use the Arduino code "Knock" to analyze the analog input values.