What is A toy in which you look into a hole at one end of a cylinder and see shifting designs as you rotate the other end?

The toy you are describing is called a "spirograph" or "viewfinder." It consists of a cylindrical body with a viewing hole at one end and a rotating mechanism at the other, allowing for the creation of intricate geometric patterns or designs. As you rotate the end, the internal components shift, producing mesmerizing visual effects. It’s a classic toy that combines art and science, often used to inspire creativity and exploration in children.