PUZZLEBOXX

Look at the bottom left corner of the shape. The black face is at the front of the shape. As you move right along this black face you are moving towards the back of the shape. Continue along the dark grey surface and you are still moving towards the back of the shape. But as you drop down the light grey face you end up back at your starting place - which is at the front! The illusion is caused because each of the corners appears to be a right angle (drawn in perspective) which fools your mind into turning a corner.