Long and Short
Look at this geometric shape. Which of the blue lines is longer, the one on the right, or the one on the left?
This is called the Sander illusion. Although the line on the left appears to be considerably longer, the two lines are actually the exact same length. Get a ruler out and see for yourself!
Now look at the two black rectangles. Which one is longer?
Once again, they are the same length, although our brain perceives the one on the right to be longer. This is called the Ponzo illusion.
Here is one more to try from this family: the Muller-Lyer illusion.
By now you probably guessed that the red line and the green line are in fact the same length, despite the apparent disparity. In the variation and unmasking you can see that even though the lines appear to be different sizes, they are clearly all the same.
In all of these cases, what's most likely at work is a framing effect. The objects that surround or "frame" the lines in question affect your brain's perception of the line length. Interestingly, studies have found that while these effects persist across various human cultures, they are not always perceived by birds!