This illusory triangle was first created by Swedish artist Oscar Reutersvärd in 1934. Similar impossible polygons are most familiar from the continuous staircases and waterfalls of Dutch artist M C Escher, where the different perspectives of parallel lines create the illusion of angles that are at once perpendicular and acute.
British mathematician Roger Penrose, who independently worked out and popularized the device in the 1950s, described it as “impossibility in its purest form”.
As an actual, physical object, it is impossible. However the illusion can be made in 3 dimensions by creating a shape consisting of horizontal and vertical bars joined at right angles. Viewed from the correct angle, the lines, angles and shadows create the illusion of the impossible triangle.
To make your own, simply print out the plan in the image below, cut, fold and glue where indicated.
Place it on a desk or table where visitors coming into a room will see the triangle form and disappear again, they will be astounded.