To make a stroboscopic viewer.
The stroboscope can be created by cutting eight slots at evenly spaced intervals around the border of a circular cardboard sheet. A stroboscope is an instrument that appears to slow down or stop moving objects by cutting off light at regular intervals of time. Then insert a pin through the centre, and stick the pin into the end of a thin wooden stick so that the disc can spin freely as shown in the diagram.
To use the stroboscope, see through the rotating edge at a moving object. According to the speeds of the stroboscope and moving object, the object will appear to slow down or stop. The reason is because the object is seen only at regular intervals and not continuously.