Here is a demonstration of Peaucellier's linkage, a machine which converts linear into circular motion.
Initially, you are allowed to move the red dot in any way you'd like. However, if you press "Constrain," you will see a new dot which allows you to position a line to which the motion of the red dot will be constrained. When you have the desired line, press "Line Set." You may now move the original red dot, but it will be constrained to lie on the line you choose. You will see that the cyan dot traces out an arc of a circle.