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In 2000, as the internet expanded, websites faced a growing challenge to stop spam bots from flooding their systems.

To separate humans from machines, researchers at the United States’ Carnegie Mellon University in Pittsburgh, Pennsylvania, created the Completely Automated Public Turing test.

From its early development to its evolution into reCAPTCHA it continues to block millions of automated attacks every day.

Ashley Byrne speaks to computer scientist Andrei Broder, who played a key role in developing the concepts that helped shape this technology.

A Made in Manchester production.

Eye-witness accounts brought to life by archive. Witness History is for those fascinated by the past. We take you to the events that have shaped our world through the eyes of the people who were there.

For nine minutes every day, we take you back in time and all over the world, to examine wars, coups, scientific discoveries, cultural moments and much more.

Recent episodes explore everything from the death of Adolf Hitler, the first spacewalk and the making of the movie Jaws, to celebrity tortoise Lonesome George, the Kobe earthquake and the invention of superglue.

We look at the lives of some of the most famous leaders, artists, scientists and personalities in history, including: Eva Peron – Argentina’s Evita; President Ronald Reagan and his famous ‘tear down this wall’ speech; Thomas Keneally on why he wrote Schindler’s List; and Jacques Derrida, France’s ‘rock star’ philosopher.

You can learn all about fascinating and surprising stories, such as the civil rights swimming protest; the disastrous D-Day rehearsal; and the death of one of the world’s oldest languages.

(Photo: I am not a robot. Credit: Stock image / Vector Illustration)

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