When we see a real object in our world, it's fully three dimensional, unless we are looking at a printed page or a screen. Animated gifs allow us to make 2D images move, and these images can have none of the cues we need in order to interpret the shape of the object depicted. Here's a famous example, a girl spinning about one foot. Is she spinning clockwise or counterclockwise seen from above? The amazing thing is that your brain will force an interpretation. While you have been viewing the image you have ``seen" it spinning one way. Concentrate and you will find you can reverse the spin and get the image to spin in the opposite direction from the one your eye-brain system first decided upon!

If we modify the image by sticking on crude``highlights," to create the illusion of 3D, then the ambiguity in rotation vanishes.

This works with linear as well as rotational motion. Consider this commuter train rushing through a station. While you are viewing the gif, your eye-brain system has decided the train is either moving away from you from right to left, or is moving past you from left to right. Whatever you are seeing, probably the first option, concentrate and you can get the illusory motion to change to the other interpretation! Notice that if the animation were slower and less jerky, the illusion would vanish.