Prepare to get seriously annoyed.
Take a look at the two triangles below, both composed of the same four coloured shapes, arranged differently in formation A and B.
The same four shapes, right?
SO WHY IS THERE AN EMPTY SQUARE IN THE SECOND TRIANGLE?
Well, it's actually because rearranging the four shapes creates an ever-so-slightly different triangle.
In fact - neither 'triangle' is actually a triangle. Their 'hypotenuses' are actually made up of two lines, at different angles - so they are, in fact, quadrilaterals.
Triangle A has the lines creating a 'concave' shape, while Triangle B creates a 'convex' shape.
Thus, Triangle B has a slightly larger area, which is exhibited in the extra empty square.
The gif below helps explain it.
BUT WE'RE STILL NOT SURE WE BELIEVE IT.