Inside your brain is a complex map of your world. But sometimes you don't let it tell you everything it knows. And sometimes you believe the evidence of your own eyes, when you should be letting your ears do the talking. So how do you know what's real and what isn't? The answer is not as simple as you might think. Tonight, in the second of a series of six programmes, James Burke goes to the circus, climbs Blackpool Tower, and turns upside down in order, to discover how you know where you are, and which way is down.