Hannah Fry finds out how 1970s car crashes helped us to count our steps, why WWII dogfights led to a crucial health monitor and how fitness trackers are even being used to catch killers. Show more
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Hannah Fry finds out how 1970s car crashes helped us to count our steps, why WWII dogfights led to a crucial health monitor and how fitness trackers are even being used to catch killers. Show more