I couldn't tell you why this happened or what led to the 1970 Chicago Blackhawks taking the ice with Sesame Street characters in a hockey game, but I'm so very happy that this video resurfaced recently.
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The Chicago Film Archives posted this video to YouTube recently and there isn't any sound or much of an explanation other than a guess that this was part of a promotional video. It really doesn't matter because the video of Cookie Monster playing in goal and Big Bird gooning around trying to pick a fight with Blackhawks players is just fantastic.