Why can you bring a cow upstairs but not downstairs?

Cows can go downstairs. See for yourself:
http://www.youtube.com/watch?v=sfyPOzB7pB8

If, however, one buys this myth one should know that there is an entire organization dedicated to descending one's cow safely, see here:
hyperactive-stage.co.uk/bovinedescenders/how.html webpage no longer exists

Someone at an official US government agency has spent the time compiling for us a page entitled, "Trivia and Useless Facts," from which I've copied this datum:

"It is possible to lead a cow upstairs but not downstairs, because a cow's knees cannot bend properly in order to walk down."

riley.nal.usda.gov/nal_display/index.php?info_center=8&tax_level=2&tax_subject=433&lev
el3_id=0&level4_id=0&level5_id=0&topic_id=2036&&placement_default=0 no longer exists

My grandfather had a story about a Halloween prank about 1900 or so in which the local boys got a cow up in a belfry. She had to be slaughtered up there because they couldn't get her down. If you'll notice in Bry's video, the cow doesn't descend a flight of stairs, but a couple of shallow, broad steps. I've been around Jerseys in Florida, Zebus in Colombia, and Holsteins in Alaska, and can say getting a cow to go where she doesn't want to takes time, determination, and luck. But this one seems to be more about anatomy than fear or stubbornness.