I light the pilot on my water heater, turn the knob to on and the burner ignites and heats water. When the burner shuts off, the pilot light goes out.
About two years ago the water line above the water heater was leaking badly. My husband would not fix it nor let me call a plumber. My sons finally replaced the leaking pipe. After that we had this continual problem. One of my sons replaced the thermocouple.
So what we do is light the pilot, turn the knob to on, the burner ignites and heats the water for maybe an hour, depending I think on the temperature of the water. If we want a full tub of hot water or to be able to use the dishwasher, we have to relight the pilot a few times to go through the cycle of the burner flaring to life and heating some of the water. Just one cycle is not enough to get hot enough water to start with to run the dishwasher. So to get a full tub of warm/hot water, if I didn't light the pilot at all that day, I have to light the pilot a few times over an hour or two, I start filling the tub, then go down to the basement and relight the pilot one last time. By the time the tub is full, the water coming out of the faucet is lukewarm.