I have a piece of string which I stretch out in a straight line. I then take a thin red felt-tip pen and make a small mark in the middle of the string. Then I fold the string in half and again stretch it out, holding onto the fold and to the two ends. This time I make a small mark in the middle of both strands of the folded string with a thin green pen. Then I fold it in half again and make a small blue mark in the middle of each strand of the folded string. Finally I fold it in half once more, and this time I make a small purple mark in the middle of each strand of the folded string. Now I unfold the string and stretch it back out to its full original length.