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The National Football League is America's most popular sport. It's grown into a multi-billion dollar business and the annual draft pick is closely watched around the world.

This time last year, the first pick signed a contract worth more than $30m over four years, with a $22m signing bonus. Even the last pick - the 256th to be precise - went for around $2.5m with the signing bonus cut to a paltry $70,000.

Professor Cade Massey of the Wharton School at the University of Pennsylvania, explains the nuts and bolts of the system.

(Image: The NFL draft logo is on display at the Draft Main Stage on Lower Broadway in downtown Nashville, TN. Photo credit: Michael Wade/Icon Sportswire via Getty Images)