Almost since the invention of the wheel, scientists and engineers have dreamed of inventing a perpetual-motion machine.
Never happened - probably never will - but the closest they have come is probably the Citizen Eco-Drive Watch. It is battery powered, but that battery never needs to be replaced and is constantly charged by ambient light.
I have been the proud owner of one of these amazing watches for over a quarter of a century. It is a WR100 model, guaranteed waterproof to a depth of 100 feet. I never take it off - I've even worn it scuba diving - and the only time it needs adjustment is on the first day of any month that comes after a month of less than 31 days.
It has kept perfect time - never failed - until about four days ago.
I'm not sure what is wrong with it. It's displaying a symptom similar to what the Citizen website says you might expect if the battery was recharging after the watch had been stored in the dark for a week or two. It hasn't been off my wrist, so that obviously isn't the problem.
I've sent it in for repair.
Now all I can do is hope, and keep looking at my naked wrist.