The triangle on your screen is 2D, yet your brain automatically tries to picture it as a 3D object. To do this, your visual system is making assumptions based on what it has seen before.

For this triangle, most peoples visual systems make the false assumption that the triangle is in one plane – i.e. it would lie flat on a table. But if this is true, the triangle would be impossible to build.

When you cover up the top of the triangle, you get an idea of what a model of this shape would really look like. It is not a triangle at all – in fact one part of the ‘triangle’ actually sticks up into the air.