The Knock Clock is a clock which tells the time through the medium of knocking.
It was thrown together quite quickly in about 2012 for a mini Maker Faire by Tallscreen.
There is a write-up for the project here... http://www.paulplowman.com/projects/knock-clock.htm
The clock has since appeared on the TV show 'QI'.
Tallscreen has since created a better version of the clock with a neater box, and more accurate clock, and not held together with gaffa tape!