"My Wife and My Mother-in-Law" is one of the most well-known optical illusions in the world.
It hinges on facial perception: You can either see a young woman turning away or the profile of an older woman staring solemnly towards the left side of the drawing. However, you can only see one at a time.
Image: Here's a hint: The young woman's necklace is the old woman's mouth.Wikimedia Commons
