Anti-parabola propoganda, plus musing on math class, cardioids, connect the dots, envelopes of lines, even a bit of origami.
Another way to get a cardioid is to add up circles like this: ...
by King's Fine Woodworking
by Steve Yalo
by Avneet Kaur
by CraftyStampin - Linda Cullen
by Craft Jitsu