Leaking natural or propane gas can create a serious explosion hazard. Fortunately, the pilot light on modern water heaters is controlled by a valve that closes when the pilot goes off, shutting off the supply of flammable gas.
Pilot Light
The pilot light provides a small flame to the water heater burner. When the gas goes on in response to the cooling tank water, the pilot light supplies the burner with a flame to ignite the gas. The pilot light is on all the time.
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Thermocouple
A device called a thermocouple is heated by the pilot light flame. As long as the thermocouple remains hot, gas is supplied to the pilot light. If the pilot light goes out, the gas is shut off and there is no danger of gas accumulating because the pilot light is out.
Lighting A Pilot Light
Nearly all gas water heaters with pilot lights are lit the same way. A button is pushed to temporarily supply gas to the pilot light. The pilot light is lit with a match or long lighter. Once the thermocouple is hot, the button may be released and the pilot light will stay on.