I only found out about them well into my 30s, and was so enamored by the one my friend showed me that he let me keep it. It's enchanting.

There's also the pseudoscope, which reverses one's depth perception. I can grasp the concept, but I have real trouble imagining what that experience is like. I was unsuccessful in ever finding one for sale, until just now! https://www.grand-illusions.com/collections/optical-illusion...