Imagine you are suspending a cube from one vertex (corner) and allowing it to hang freely. Now imagine you are lowering it into water until it is exactly half submerged. What shape does the surface of the water make around the cube?
What happens to the perimeter of triangle ABC as the two smaller circles change size and roll around inside the bigger circle?
Draw a square. A second square of the same size slides around the first always maintaining contact and keeping the same orientation. How far does the dot travel?
What path does the centre of the circle take as it travels along one side of the shape?
What shape will the path of the centre be as it goes round a corner?