By the 1930s, the horses had been replaced by cars with the completion of the aptly named Going-to-the-Sun Road, a heart-in-your-mouth rollercoaster that climbs past cascading waterfalls and gaping cliffs up to the continental divide at 6, 646ft-high Logan Pass.

BBC: Glacier National Park by train

When one of the orang-utans displayed an open, gaping mouth, its playmate would often display the same expression less than half a second later.

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Facial expressions, such as the open, gaping mouth resembling laughter, were picked up and copied by orang-utans.

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