Serena Williams has nothing left to prove. She's won 23 Grand Slam singles titles, inspired generations of players and built a legacy that reaches far beyond tennis. But after nearly four years away from the sport, she's returning to Wimbledon.

So, Niall asks why one of the greatest athletes of all time is making a comeback. He is joined by Charlie Eccleshare, senior tennis writer for The Athletic, as they discuss what motivates elite competitors long after they've conquered their sport, and whether Serena Williams can still defy expectations one more time.

Have you got a question for Niall? Email the show: why@sky.uk. And if you wanted to watch Niall, check out This is Why on YouTube.