Spinning Illusion

Spinning Illusion

Here is an interesting optical illusion that you can make!Fold an index card (or a small piece of heavy paper) in half both ways. With a heavy marker, draw 4 lines starting at the top center of the index card, as shown in the picture.
Carefully poke a thumb tack through the center of the index card from the bottom.What will you see if you spin the index card on the thumb tack? Try it and see! Somehow the straight lines turn into a circle within a circle when you spin the card!