Why can a fuel cell operate continuously when fuel and oxygen are supplied?

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Study how a series of gears in a waterwheel translates a stream's energy to a millstone — energy conversion Before the Industrial Revolution, power came from three main sources: humans, draft animals, and water.
The ingenuity people used in harnessing waterpower can be seen in this medieval-style mill. The waterwheel is turned by a stream and is connected to a shaft that leads into the building. At the other end of the shaft is a gear. The connection of a series of gears translates the power from the stream to a shaft that drives a millstone, which grinds flour from grain.
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A fuel cell can operate continuously because, unlike a battery that contains a finite amount of reactants, a fuel cell is designed to be supplied with fuel and an oxidizer from an external source.12

The core components of the system—the anode, cathode, and electrolyte—are engineered to remain unchanged by the chemical reaction itself. As long as fuel (such as hydrogen) and an oxidizer (such as oxygen) are provided, the cell continues to convert chemical energy directly into electricity. The process relies on the continuous flow of reactants to maintain the electromotive force required to generate electric current for an external circuit.12