Kim changed her furnace thermostat, only to watch the wires all fall into the hollow wall as she unhooked the last one. What a job to get them back.
So she advises us to wrap the first loose wire around a pencil before unhooking the rest, that way they will stay on this side of the wall.
With thermostats, the wires, as well as the screw terminals are colour coded to a furnace standard. It is fairly simple to change thermostats because you attach the yellow wire to the Y screw, and so on.
If you are undoing wires, like for a ceiling fixture, and there may be a lot of confusing wires in the box, make a drawing of all the wires before undoing any of them. Then tape labels on them, simply 1, 2, 3 and put those numbers on your drawing. When you put the new ceiling fixture up, you don't have to worry about mixing up any wires and blowing fuses.