It's important to make sure you have the right perspective, because even the real world can turn into an optical illusion.

While optical illusions can be fun little games to look at in your spare time, they can also exist all around you. These 23 photos were taken by people who had to do a double take on real life. Perspective is of the essence in this funky photo gallery.

Hotel floor pattern.

These are the same photo side by side.

When a helicopter's propeller is perfectly synced with the camera frame rate, the result is always astounding.