I was searching for brain teasers and found optical illusions. I hadn't thought of them as brain teasers, but some do test your ability to think (view) things differently.
This one is an old classic that everyone has probably seen. Do you see a vase or two faces in profile? I easily see both and one doesn't dominate the other for me, maybe because I've seen this so often.
If you had trouble seeing the young girl, think of the image as being done from her side with her looking away from the artist/photographer. A small protrusion defines her nose, the larger one is her jawline. The line above the collar of her dress might be a necklace. Her ear is detailed near her long, flowing hair.
To see the old woman in the same image, what was the young girl's ear is the old woman's eye. The young jaw is the older woman's nose and the necklace is her thin mouth. The hair becomes wrinkles near the eye and I'd like to think those are laugh lines near the mouth.
I think the donkey/otter is an easy one but a friend did not see the otter until I pointed it out. The details of the donkey's snout becomes the otter's head. The donkey eyes are the fins and the donkey ears are the tail of the sea animal.
Still stuck on that word? It is Eureka.