A black bear and her three cubs chased a group of petrified tourists on a bridge in Yellowstone National Park, which has the black bear as its symbol.

A video showed the bear bounding along the bridge with her cubs keeping up as the tourists tried to keep a safe distance, yelling, ¡®Go, go!¡¯ while some people on the bridge kept taking photographs, even as the bear veered in their direction.

Bears are highly protective of their cubs and will attack if they feel threatened. People were not always as cautious of bears in the park – from the 1910 to the 1960s, Yellowstone Park managers allowed visitors to feed the black bears along the roads.

Difficult words: cub (a young bear), petrified (very scared), bound (walk or run with big steps), veer (change direction suddenly), threatened (in danger), cautious (careful).