This is the inquiry on the BBC World Service with me Ruth Alexander.

Each week one question, four expert witnesses and an answer. It was late afternoon on the 24th of November 2017.

Black Friday, a big discount day in the shops, and one of the busiest days of the year for

London's famous shopping destination, Oxford Street.

Oh my God, they're actually running. Hundreds of men, women and children poured out of Oxford Circus Station,

gripping shopping bags and one another's hands. They were running for their lives.

Some were screaming, some crying, some were on their phones to the police, reporting hearing gunshots.

Armed officers soon swarmed the area in search of the attackers, while shoppers were barricaded inside stores.

After an hour, the anti-terror operation was over.

The police had found nothing.

No terrorists, no victims, no guns, no bullets.

Later the police said they believed there'd been an argument between two men on one of the station

platforms, hardly a reason for math panic. And yet in a city on edge after two

genuine terrorist attacks only a few months earlier, the fear people had felt, as rumors

of violence had spread through the crowds, was real.

Locked inside the BBC building just up the road throughout the non-incident,

it had made me curious about what makes us fearful.

What happens when we're in the grip of fear,

and whether it's an emotion we need to get under control. So in this

week's inquiry I want to know how scared should we be.

Part 1, Fight or Flight.

When I did the CN Tower Edgewalk, when I did the CN Tower Edgewalk, which is in Toronto, it's the tallest structure in the Western Hemisphere,

...