On hackaday.com I came across the extremely clever design of a knife by user @uhltimate_548874. It can be mechanically opened and closed with the thumb.

On »OTF Fidget Knife - only 3 Parts von uhltimate« for printables.com it is described as follows.

This compact design is perfect for restless hands, providing smooth OTF action for endless fidgeting fun.

The text was automatically translated from German into English. The German quotations were also translated in sense.