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](user:tallscreen "wikilink").

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](user:tallscreen "wikilink") has since created a better
version of the clock with a neater box, and more accurate clock, and not
held together with gaffa tape!