how does a GUI slider work? i tried this:

var Number = 10;
function OnGUI () {
Number = GUI.VerticalSlider(Rect(10,10,40,200),Number,20,0);
print(Number);
}

but it wouldnt work, any reason why?

(in otherswords, how do you use a GUI slider, noob question i know but i cant find it ANYWHERE, and yes i did read the wiki and the runtime class available on resources)