Every oven I’ve ever used has had a secret function, a mechanism that automatically tells you when the food is ready. It is wonderful and I want to tell you about it.
So most ovens control their temperature using a bimetallic strip. When the temperature inside is less than the target temperature, the strip closes a circuit that activates the heating. As soon as the temperature is sufficiently big, the strip will have deformed enough to open the circuit and stop the heating. In many ovens, especially older ones, you can hear this as a soft *click*. If you are lucky, the mechanism is sensitive enough to rapidly go on and off to stay on temperature, at least for a couple seconds.
If you eat frozen pizza, it often only has to be heated to a sufficient temperature. When it reaches this temperature, the pizza will stop cooling down the air around it, thereby allowing the oven to reach its target temperature and starting to say *click*. So the sound will tell you when the food is ready, no need to read the packaging to find the correct baking time.
The same happens for dishes that are ready when enough water has evaporated, or when a certain endothermic chemical reaction has stopped happening. All are done the moment the oven says *click*. There might be some exceptions to this phenomenon, but I have yet to run in to one. Which is great because I always forget to read oven instructions on packaging or recipes before throwing them out. Try it out with your own electrically powered food heating units.