Knock Knock is a playful calculator made for children by designer Khalil Klouche. The interactive wooden box lets you “knock” out math problems by using the standard arithmetic symbols displayed on the device. Using an Arduino board, contact microphones, and solenoids, you “knock” the math problem and it sounds the answer back to you.

Watch how:


For example, say you want to subtract two from a number, you knock the ‘minus’ symbol two times and it will then give you the answer in a knocking sound.