An illusion I am sure you have seen before, but always impressive when presented in a form that actually works. It shows how the size of an object is partially based on the context in which it appears. In both images, the size of the central blue disk is identical. It does not appear that way!
Here's a nice example of what's sometimes called a ``bleeding color" illusion. You should see the central region of the figure as a faint blue disk, with the black threads turning blue as they pass in front of the disk. Actually there is no blue disk. The color change of the threads ``spreads" but the background is always white.
Here we have a cat. What's the illusion? Question: is the cat climbing up the stairs toward you, coming from the floor below, or is the cat climbing down the stairs toward you, coming from the floor above? Are there any clues in the photo as to what was actually happening when the photo was taken?
Now try this one, which is I believe my all-time favorite illusion. This interactive page may not work on some browsers.