Richard Grove, 73 years old and a robust 6 feet tall, set out with confident strides across a laboratory floor the other day. His first five steps went great.

Then his left foot hit a slippery patch and skidded. His arms windmilled over his head as if he were throwing a baseball with each hand. His right foot shot forward to come even with his left. But he quickly regained his balance and kept on walking.Read this article in full with a
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