Here’s an ingenious contraption:
The creator, Lior Elazary, provides a full explanation of the clock here, along with instructions for building your own. Here’s my favorite part of the instructions:
Start by creating a 12” diameter disc and attach a Flip-Flop to it. A Flip-Flop is a device that alternates its state with a given input. For example, a politician might change his stance on a specific issue based on some event, and then will change it back based on another event. In that case, we say he is a Flip-Flop, and we might be able to build a computer out of him.