As an introduction of drawing parabolas from focus and directrix, I had the students create a parabola from patty paper, but folding the directrix on top of the focus. The envelope of the folds creates the parabola. I didn’t like it very much as an intro. Yes it’s cool that it makes a parabola, but the math behind the scenes is tougher to explain. Next year I’ll go back to the human parabola (have students stand in the classroom, equidistant from a focus and a wall).