What did you discover
With your eyes, you see the two drawings one after the other, in quick succession. But because this happens so fast, your brain no longer understands that they are two separate images. Instead, it turns them into one image. A film or video works in exactly the same way. It is actually nothing but a whole series of still images you view in rapid succession. Your brain has no time to realize this, though, so it merges the images together and makes them seem like one moving scene.
Scientists discovered this optical illusion more than 150 years ago, and invented all kinds of devices to demonstrate it. The thaumatrope is one of them. “Thauma” means miracle in Greek. And “trope” comes from trepo, meaning to turn or spin, also in Greek. So: a spinning wonder.