I like optical illusions. Look at the red and blue hearts and examine their depth with respect to the background.
Most people find that the red heart pops in front of the blue background whereas the blue heart sinks beneath the red background.
This illusion comes about because the lenses in our eyes refract (bend) blue light more than red light. This phenomenon is called a chromatic aberration - which basically means that our lenses fail to focus all colors to the same point. The brain deals with this sensory aberration by imagining depth—the red heart is in front of the blue background and vice versa—even though none actually exists.
