Pocket Caliper, For Precise Measurements Of Interesting Things

These calipers use 2mm increments. Each bump is 2mm, and each valley is 2mm. There are vertical strips running down the left side, which indicate tens. So a quick glance can tell you immediately that an object is both 15mm AND somehow already your fault.

This can be used with one hand, and functions as a fidget spinner when you have run out of things to measure.