This is one of the most incredible illusions I have ever seen. It requires some thought as to how it was done, and as to whether it’s possible at all!
However, it is possible, and I’ll show you how to do it now!

Supplies

You’ll need a strip of paper, scissors, and a pencil.
Use the image to trace the line onto the paper by placing your strip of paper on your iPad or laptop, turning the brightness all the way up, and trace.
(This is a very useful hack, so remember it!)

Step 1: Cut Out the Line

Use your scissors to carefully cut out the pattern on your paper. Guide the scissors over the line. You will be only using the bigger side of the paper for this project.

Step 2: Tape and Squash

Curl up the paper so the top and bottom edges line up. Then tape this in place on both sides.
After you do this, squash the paper slightly perpendicular and in line with the tape. Make this as exact as possible. You know you did it right if you look from the top view and it is between a circle and a square.

Step 3: Place in Front of a Mirror

You can put this in front of a mirror so you can see both sides at once. IMPORTANT NOTE: you may have to trim the sides based on your camera’s focal length. If you don’t know the difference between the sides and the back and front, use trial and error. You did it right if the resulting illusion looks like the attached photos.