A picture of stacked unit cubes hides blocks behind the ones you can see from the front. Checking the number stacked in each square with a number grid lets you count exactly.
Pictures of stacked cubes can trick the eye. The floor is divided into a grid like a checkerboard, with blocks stacked on each square — and a block hidden behind a visible one can be easy to miss. Counting from the picture alone makes it easy to overlook the back.
That's why looking at a "number grid" — the count stacked on each square written as a number — is much more accurate. For example, if a 2×2 grid shows 1, 2, 2, 1, the total is 1+2+2+1=6. A staircase shape that's tall in front and gets shorter toward the back is especially easy to miscount, so it's important to double-check with the number grid.
First guess the total just from the picture, then reveal the numbers one at a time and check whether your guess matches the real count.