Multistable Objects are perceptual phenomena when unique image can't be recognized from ambiguous patterns.
The most well known is probably spinning dancer illusion which is often erroneously used as brain test to determine whether right or left hemisphere of brain is dominant.
This corner house is a bit tricky. Question: Is the corner concave or convex? Use your palm to cover the upper half of the picture and the corner will be bulged out. However, if you cover the lower half, then it will seem as the inner corner. Amazing.
At first sight, you probably see just 1 face in this Venetian mask. However, there are actually 2 faces - a man and a woman kissing one another. Once you spot them, you will be able to flip between both perceptions of this bistable image. (by 2011 Gianni Sarcone, Courtney Smith & Marie-Jo Waeber)