There are many types of optical illusions. Some of them are:
- Stereoscopic images: if you cross your eyes lightly, you can mix two similar images to create a 3-dimensions one.
- Autostereogram: if you cross your eyes standing 30 cm in front of these images, you'll see as if it had 3-dimensions relief. In the first one you'll see a heart, in the second one, a shark.
- Motion illusions: if you move your head towards and backwards the picture, looking at the point of the centre, you'll see the two circles move.
- Other effects: In the first case, all the lines are pallalel, even though they don't seem to be. In the second image, the two circles are the same size.