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Three-legged race
If your kids are fighting, set them up for a three-legged race and they'll have to stay united. Apparently a Hispanic tradition, this race where kids tie their legs to each other and run a course is so much fun. It's great when a team share the same stride (twins might be great at this), but it's also pretty hysterical when a pair can't go more than a few feet without falling over.

© U.S. Air Force photo | Roland Balik