Take a long look at the Mona Lisa. How do you see her? As blobs of paint or as a woman with an enigmatic smile? Now explain how you came to see those blobs of paint as a smile. For your second mission, think back to learning to form sentences. Your parents never told you "verb in the middle" (if you're English) or "verb at the end" (if you're German) but still you picked it up. And, more remarkable, once you did, have you any idea how come this sentence breaks the rules but read it you still can? These abilities demonstrate what's known as "tacit knowledge" - something as big and taken for granted as "air", "thought", or "language". Take away tacit knowledge and the human world disappears. Without it, what we think of as knowledge, the "stuff" contained in our books and intellectual artefacts, would make no sense and be no more than noise. The big question is whether, or how far, this tacit knowledge can be made explicit.