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Thermocouple. Keep pilot knob depressed for 30 seconds after lighting pilot.
Gas not turned on to water heater. Gas valve on water heater not set to pilot setting. Faulty gas valve. Igniter for lighting pilot not working.
The pilot controls the helicopter.
Think it is just that the pilot light is lighting the main burner, which makes a booming sound.
Push the button down and hold it while lighting.