Take a band of paper, say a postal wrapper; you observe that it has two lines and two surfaces (interior surface and exterior surface.) The problem is to arrange it so that it presents only one line and one surface. It may seem improbable, yet it is possible as you will see. Cut the band and gum together again the two pieces thus separated, after having turned over one of them as shown in figure as above. Arranged in this manner the paper has but one line and one surface, for it has the aspect of a screw without end.