- A file card
- A felt tip pen
- A long pin
On a file draw four thick black lines as in the illustration.
On the card's centre pierce a long pin.
Now hold the pin head beneath the card, and spin it fast by waving it with the other hand.
What do you see?
HOW DOES IT WORK?
What will be seen are two concentric black circles! And not many make a correct guess.
It is the same explanation as for "The Moving Circle" (refer to the contents). Once the card rotates, there is a spot on each line which travels a shorter length, in comparison to other spots on the line. You will view the four spots a mite longer. And they form circles.