Extreme Science: Penny Shine

What you will need:

  • Toilet paper tubes or similar cardboard tube.
  • Pencil
  • Scissors
  • Ruler or straight edge
  • Mirror
  • Adult supervision


  1. Lay the empty cardboard toilet paper tube flat on the table
  2. Press it flat as shown
  3. Now, open your tube back up and line up the two creases inside the tube that you just made when you pressed it flat on the table. Press it flat again so that you will now have four creases inside of your tube as if you were trying to change its shape from a round tube to a square tube as shown.
  4. Keep your tube flat on the table and mark a line about ¼” – ½” from one end of your tube as shown
  5. Draw a curved line as show across the end of the tube making sure that it begins, ends and crosses the straight line that you just made where it intersects at your creases (Identified by yellow circles in drawing below). Since the curve needs to be the same on each side, you may find it easier to use a round container as a guide.
  6. Keep your tube pressed flat while you cut the end of your tube along this curve line as shown.
  7. Now open your tube and stand it up with your cut end upward as shown. You will notice that when you look at your tube at just the right angle, it will either appear to be square or round, depending on which side you are looking at. When you place it in front of a mirror you will now be able to see BOTH the round image AND the square image from the same item. Rotate the tube 180 degrees and the images will be swapped – the one that appeared round now appears to be square and visa versa.

What is going on?

When you began this project, you had a cylinder (cardboard tube) that was cut straight on each end. As you held your tube at a slight angle and looked at the cut end, you could easily see the round cylinder shape of your tube. By pressing your tube flat and making the four (corner) creases down each side, you now have what some would call a “squircle” which is a blend of a square and a circle. By cutting the top as we did with an “S” curve, rather than keeping the top of your squircle flat, you create what is known as an optical illusion. Now, depending on what angle you look at the top of your tube, you will either see a circle (when the back corner dips down and the front corner points up) or a square (when the front corner dips down and the back corner points up). If you took a round dowel and placed it through a square dowel at an angle, the top of your squircle would actually follow the intersecting line between these two dowels. As with the dowels, depending on which angle you look at it, you will either see the end of the round dowel giving you the appearance that it is a circle, or a square dowel giving you the appearance that it is a square.

Try this:

You can also make the inverted carport illusion that was shown on air by following this link:


Please know that this is NOT MY WORK, I am only providing the link to a very cool project that SOMEONE ELSE created. I do not take credit for this in any way.

Radical Rick

Extreme Science