Get hold of a screw and a screwdriver and try screwing the screw into a piece of wood (make sure it is a scrap piece of wood). Watch it as it goes in. How does it work? What happens if you turn the screwdriver the other way? Why? Does it matter which way you turn it at first? Why?
If you cut open the wood right across the screw hole, what do you think you will see? Now cut it open - were you right?
Now try working with a C-clamp. What happens when you spin it around? Why? How about when you spin it the other way? Does it matter how long the turning stick is? Why?
Compare several different C-clamps. Are the threads the same distance apart on each of them? Are the heads the same thickness? Are the sticks the same length (or can you make them longer by slipping metal pipes over the ends of the sticks?) Which clamp will take more turns to close? Which will hold on the strongest when it is closed? Why?
Bibliography and further reading about machines: