This is another classic optical illusion. The name for this is the Ponzo illusion and has a lot to do with our perception of distance and using cues from perspective to judge an item's size.
Enough chitchat for now. Look at the image below and guess which yellow line is longer.
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Step 1: Get an Image With Perspective
OK, all images have perspective, but we want one where many lines are converging to the same vanishing point. In this picture that vanishing point is right above the end of the escalator.
The source image can be found here on Flickr.
Step 2: Add a Yellow Line
Now, on a new layer, add an yellow line low on the picture.
Step 3: Copy the Line
Copy this line and place the copy directly above the first one and just a little below the vanishing point.
Step 4: Try It Out With Other Images
Step 5: A Simpler View
If you don't fancy pictures of tracks and roads, you can just use black lines instead. See below for this effect.
The effect works because we're tricking our own sense of depth perception. Items that are closer to the horizon are farther away. So naturally if the yellow bar is closer to this it looks bigger even when it's exactly the same length.