Check out this cool circle optical illusion.
The white balls appear to be moving as a circle rolling inside of the lager circle.But what you see is actually not true. This is an optical illusion.The true path of each ball is a straight line. The mechanical design was made by Girolamo Cardano. Cardano’s work with hypocycloids led him to the Cardan joint or gear mechanism, in which a pair of gears with the smaller being one-half the size of the larger gear is used converting rotational motion to linear motion