Here's a fun experiment to try next time you find yourself in an open field. Close your eyes (or wear a blindfold) and set off walking, taking care to walk in as straight a line as possible. Have a buddy monitor your route (and warn you of obstacles). Sounds simple enough, right? Turns out it's harder than you think.
In this video, NPR's Robert Krulwich reports on a puzzling bug in human proprioception, viz. our inability to walk in a straight line without some point of reference:
There is, apparently, in humans a profound inability to stick to a straight line when blindfolded, or when there is no fixed point –no Sun, no Moon, no mountaintop – to guide them.
For eighty years, scientists have been trying to explain this tendency to turn when you think you're going straight... But try as they might – and we're still trying these experiments – nobody has really figured out why we can't go straight.
As Krulwich explains, the effect carries over to driving and swimming, too. If you've ever swum in open water, chances are you've experienced this first hand.