Ko-An is the Japanese pronunciation of the Chinese characters:公案。
"A paradoxical anecdote or riddle, used in Zen Buddhism to demonstrate the inadequacy of logical reasoning and to provoke enlightenment." That's a brief definition of Koan, not "Koan" itself. "Koan is a fundamental part of the history and lore of Zen Buddhism. It consists of a story, dialogue, question, or statement, the meaning of which cannot be understood by rational thinking but may be accessible through intuition." That's another functional description of Koan, not "Koan" itself.
Koans could be part of your intellectual R & D department, if you treat them this way. They would become one branch of modern knowledge or a perfect historical source, but still have nothing to do with your life, even your mind.
Koans look like a powerful word-game; and that's the trick. If you want to play this game, the first thing you have to do is transcending the words. Those words are signals, marks, forms, ... etc. on the road. They point to your mind; yet not Your Mind at all. They are wonderful tools; but not destinations.
"Zen Buddhism" or "Chan Buddhism" is fun. You read in order to unread; you use words in order to get rid of words; then after all this or that, you use even more words to communicate 禪 with other people. The whole process is another Koan which you have to discover and realize by yourself.
All by yourself.