Want to get extra-silly this weekend? You're in luck. The Cardboard Tube-Fighting League (yes, there is such a thing) is hosting its 2011 tournament this Saturday at that oasis-within-the-city Hayes Valley Farm. The league, which "was created out of a desperate need to better train and arm citizens with cardboard tubes" is asking S.F.'ers to join in the fun, dress up like Medieval knights and crusaders in homemade cardboard armor, and get to dueling! In addition to the fighting, there'll be a costume contest, tours of the farm, cardboard-themed artifacts as prizes, and, naturally, lots of utterly embarrassing (but amazing!) moments. The event is free and the "swords" (aka, cardboard tubes) are generously provided.