awww brilliant. Never thought of it that way, makes a great deal of sense.

an nmos is almost bilateral. In theory you could switch the drain for the source and still get the same results. In practice there is a tiny chance that the manufacturer didn't set it up that way. However, that is for discrete in vlsi that isnt an issue.

If you remember the mosfet is made out of one material either p-type or n-type. With a body connection made out of the opposite. This means that the nmos will have a very low input current and is mostly a voltage controlled device. The nmos regulates current through it by its body voltage. Now if the source is the same voltage as the drain no current will flow but if the drain is at a higher potential then the source current will flow.

The principle at work here is a sorta dynamic resistance where Vdrain - Vsource = Iout*Rnmos

if you imagine the resistance to be steady you can see that Iout depends solely on the voltage differences, in practice Rnmos is dynamic and will work to keep Iout the same regardless of the difference in voltage levels. However, it has a limited range of operation.