Sometimes our eyes play tricks on us. Michael Bach has an extensive collection of 141 optical illusions, along with explanations of why your visual system is tripping up:
Optical illusion sounds derogative, as if exposing a malfunction of the visual system. Rather, I view these phenomena as highlighting particular good adaptations of our visual system to experience with standard viewing situations. These experiences are based on normal visual conditions, and thus under unusual contexts can lead to inappropriate interpretations of a visual scene (=”Bayesian interpretation of perception”).