Roll the circle by dragging its hub horizontally (left or right) with your finger or the mouse. The blue drawing dot will trace a curve known as a trochoid.
Use the mouse to move the blue drawing dot inwards or outwards on the circle radius. If the blue dot is on the circle, a cycloid is drawn. If the blue dot is inside the circle, a curtate cycloid is drawn. If the blue dot is outside of the circle, a prolate cycloid is drawn. (Each of these three types of curves are trochoid curves.)
Clear the curve by tapping or clicking on the "erase curve" button.