Mathematical Koan

A koan is a story, dialog, question, or statement generally containing aspects that are inaccessible to rational understanding, yet that may be accessible to intuition. A famous koan is, "Two hands clap and there is a sound; what is the sound of one hand?"

After you've pondered that for a while, try this true/false math quiz koan:

Determine whether the following sentence is true or false:

"This sentence is false."

(If that isn't hurting your brain, you answered incorrectly.)