Parameterize a cycloid: the path of a point on the circumference of a circle of radius a as it rolls without slipping along the positive x-axis.
Parameterize the path of a point at distance b from the center of disk of radius a as it rolls without slipping along the positive x-axis.
Parameterize the path of a point on the circumference of a circle of radius a as it rolls without slipping around another circle of radius a. You may want to experiment with coins to understand the behavior.