Look at the two long lines below.
Which one is longer? Try moving the two lines on top of each other. Try swapping the end pieces around.
Press the reset button if you want to start again.
The top line certainly looks shorter. Actually, it is the same size as the bottom line. Check it by moving one line over the other or getting a ruler and measuring it! This is one of the most famous visual illusions.
If you want to know more about the Muller-Lyer Illusion, check the explanation.