Where is the green side ? On the front of the cube or at the back ? Keep looking at the cube and you'll find that it switches a few times. First described by professor L. Necker who noticed this while studying drawings of crystals.
There's more in this picture than you'd think. Depending on how you look, you can see : a room with a box in it, a cube with a smaller cube cut out, or a big cube with a smaller cube in front of it.
Is the pie of cheese cut out, or does it lie in front ?
This is an animation of a rotating hollow mask. At the moment you see the back, so that the hollow side is pointing to you, something surprising happens. It doesn't look hollow, it looks like a normal mask - rotating the other way ! The most probable reason is that our vision doesn't accept hollow faces.
Is the front of the stair red or orange ? Try to see both colors as front. You'll see the stair "shift" as the other color becomes the front. This figure inspired Escher to make "Convex and Concave".