Not Gates

In electronic logic, a gate is some device that applies logic to its inputs.

An And Gate has two inputs. If both are on, the output is on. Otherwise the output is off. This means if the output is off you can't tell exactly what the inputs are - it could be that either is off or that they both are. You lose information.

An Or Gate is similar, except that for this Gate you lose information when the output is on: then you only know that at least one input was on, but you don't know which and it could have been both.

A Not Gate has a single output, the opposite of the input. No information is lost: if you know the output you can figure out the input. You might use another Not Gate to do that. The output from the second Not Gate is the same as the input to the first Not Gate. So feed the second output back into the first input, and you have made a memory device. Most transistor and integrated circuit memories use some form of this trick. They need Not Gates because Not Gates preserve information.

Your computer has maybe 0.5 billion bytes of memory? That is 4 billion bits, or 8 billion Not Gates. It is written into the hardware billions of times over: Not Gates to preserve information. And yet you boot up Windows.