Water Heaters - How does the hot water tank not overflow?

Hi, how does my hot water tank not overflow? i dont see a level sensor or any soleniod valve in the water line running to the tank.. so how come there isnt water always draining from the overflow? sorry for this stupid question.

On a very basic level, you can think of you hot water heater as just another (though very large diameter) "pipe" in your plumbing system - like all the other supply plumbing it's always completely full of water, but water only moves through it ("has someplace to go") when you open a faucet or when some other demand for hot water causes water to flow out of the system at the demand point and flow through the water heater.

It's possible that if there is a malfunction in the water heater water pressure within the heater (and within the entire plumbing supply system) could rise to dangerous levels. To prevent that from happening there's a device called the temperature pressure release valve which allows water to flow out of water heater tank if temperature or pressure rise too far - that's the "overflow" tube you see coming down the side of the hot water heater, and that's why you normally don't see water coming out of it, which will only happen if the water heater malfunctions.