Descartes' Fly
There is a legend that the French philosopher and mathematician Rene Descartes invented the coordinate plane, or Cartesian plane.
He spent much of his time as a child in the 17th century in bed.
Here he would watch flies land on the ceiling, and being unable to get up, he wanted to describe the fly’s location to others.
Move the fly around the ceiling, and describe what you see.
This is another one of my favorites for its simplicity and backstory. Students discover the origin, axes, and coordinates with this applet.