When creating models or sculptures, you have to use your hands or other tools to create them. Also, while these are being created, the material used for it tends to be pliable. Take clay for example. You can shift or shape it however you like while it is in the process of making.
(Sorry if my explanation isn't great, but it is Modeling)
Line. The most basic element of art is line, and if you look at a painting by Van Gogh, you will see that he used dabs and strokes of paint to show emotion and movement.
