Only the most observant people can figure out the difference between these triangles

These two triangles are both made up of four coloured shapes. But in the second triangle there is a missing square

Krauss/Creative Commons

This gif confuses things even more

Krauss/Creative Commons

So how does the missing square puzzle work?

The missing square comes about because the slopes on the two triangles are not actually straight

Krauss/Creative Commons

In fact, the shape of the puzzle is not even technically a triangle - it's a quadrilateral - because the "hypotenuse" (the longest side of the "triangle") is bent.

As Brightside explains: "In reality, this is not an optical illusion, but a really interesting task. The areas of the colored pieces are of course equal (32 cells), but even if both figures look like triangles of 13×5, it isn’t so. In fact, they have different areas (S 13×5 = 32.5 cells). So the mystery of the puzzle is that the first figure called a triangle is, in fact, a concave quadrangle."

This picture of the red and blue triangles helps to highlight the illusion.

As does this gif from the Russian website Novate:

Novate.ru