A cool science experiment using only paper, pen and scissors
Paper, marker pens, scissors, masking tape
Cut the paper into a long strip about 2cm wide. Ask the Beavers if they think it is possible to draw a line on both sides of the paper, without taking the pen off the paper.
Prove that it is possible! Hold the strip out straight. Give it a half twist (180 degrees) and tape the two ends together. Hold the edge of the strip against the tip of a pen. Draw a line down the centre of the strip. Do not take the pen off the paper. Turn the paper and keep on drawing the line (I found it easier to rest the paper on a table while doing this). Do not stop until your line meets up with the starting point. Take off the tape and open out the paper. Voila! A line on both sides of the paper.
Stick it back together again how it was, with a half twist. Ask the Beavers what will happen if you cut along the line you have drawn. They will probably guess that you will make two hoops of paper. What actually happens is that you make a chain that is twice as long as your original loop.
This shape is known as a Mobius strip. When you twisted the strip, the inside and outside became one continuous surface. When you cut the strip, it became one longer chain but it still had only one continuous surface.
Try the experiment again, this time giving the paper a full, 360 degree twist. This time, you'll find that the line only appears on one side of the paper and that cutting produces two interlocking loops.
Mobius strips have been used as fan belts in cars, conveyor belts in factories and continuous loop recording tapes (it doubles the playing time of the tapes).
Video demo Link: https://www.youtube.com/watch?v=BVsIAa2XNKc