The trick is to count the squares, they are the unit of measurement.
The orange shape and the light green shape are similar, but the orange shape has its thick part two squares long and its thin part three squares long.
The light green shape has its thick section three squares long and its thin section two squares long.
If you stack them with the orange shape precisely on top of the light green one, they fit neatly together.
If you move the shapes over one square to the left and down one square, they now fit with a gap because the two thinner sections are different lengths.
So why do the four pieces combine to form two identical triangles, with a square missing from one triangle??
The answer is they don't. It is an illusion. The red triangle is 8 long and 3 high, giving a slope of 8 in 3 or 2.66666. The green triangle is 5 long and 2 high, giving a slope of 5 in 2 or 2.5. So the sloping side (hypotenuse) of both triangles looks like they form one line but they don't, the green triangle is steeper than the red one.
The extra space comes because the overall shape bulges inwards slightly on the first shape and outwards slightly on the second shape. The difference is equivalent to the square.
The overall "triangles" appear the same but they are not. In fact there is NO overall triangle, it is actually a 4 sided figure. The hypotenuse of the green triangle and the hypotenuse of the red triangle are NOT in line, they form two sides of the overall shape. The OVERALL surface area of the two shapes are exactly the same, even though one looks like a regular triangle and one looks like a triangle with a bit missing. Remember, the overall shapes are NOT triangles at all.
Chris.
Thanks Louis, what a great puzzle!!
I love these sort of brain teasers.