To make a parabola with ruler and set-square.

Tape one end of a string to the pointed end of a set-square (see Figure). Tie it around a board-pin pinned to a sheet of paper. Place the set-square on the paper and set a ruler firmly against it. Hold the ruler in place and pull the string using a pencil. As you slide the set-square along the ruler, the pencil makes a parabola on the paper.