One of the most famous problems in the history of mathematics is how to divide an angle drawn on a piece of paper into three equal parts using just a compass and a straightedge (that's a ruler without markings). Of course, you can easily measure any given angle using a protractor, divide the number you get by 3, and then measure off the desired third of the angle using the protractor again. But the point is that the protractor is prohibited — you're only allowed a compass and a straightedge. The problem goes back to the ancient Greeks, who did a lot of their geometry using only these two tools.
To get a flavour of how this sort of problem might be solved, let's start by dividing a given angle into two equal parts, rather than three.