A "Monjolo" is a water-powered hammer made from a log and some sticks. It relies on flowing water from a stream to do its work.
In the video below, the anonymous laborer who goes by Primitive Technology on YouTube creates his own Monjolo from scratch. It's effectively a hollowed-out log placed in the path of a stream, supported by a structure of skinny beams. As the log fills up with water, it rises, then the water drains out the back and it comes crashing down again. When it crashes down, that's an opportunity for a hammer head on the end to do something useful—like crushing charcoal or grain.
The creator of Primitive Technology writes:
This is the first machine I’ve built using primitive technology that produces work without human effort. Falling water replaces human calories to perform a repetitive task. A permanent set up usually has a shed protecting the hammer and materials from the weather while the trough end sits outside under the spout. This type of hammer is used to pulverise grain into flour and I thought I might use one to mill dry cassava chips into flour when the garden matures. ...
Like all the Primitive Technology videos, this is done entirely without spoken or written language, and it's DIY paradise. Tune in for a look into what one man alone in the bush can create: