Kate Middleton had the best comeback when told Prince William wants to run a marathon

Banter.

Kate Middleton comes across like she would be a good laugh (remember this side-eye) and has been known to egg her husband William on when they're involved in competitive tasks during their many royal engagements. The Duchess of Cambridge gave a sneak peek at her sassy side yesterday during an event held in support of Heads Together, an organisation focused on opening up the conversation around mental health. Kate, along with Prince Willian and Prince Harry, were pictured at the Institute of Contemporary Art as Heads Together outlined their upcoming plans ahead of the 2017 London marathon. Sam Fletcher, a journalist, and broadcaster who works for Good Morning Britain, was also at the event and chatted to Prince William, who spoke about how he plans to run a marathon at some point. However, when Sean told Kate about William's plans, she responded with:

"I'll believe it when I see it".

No flies on Kate. According to Hello magazine, Sean is running the marathon to support Heads Together after his son was recently diagnosed with Obsessive Compulsive Disorder and he had only good words to say about the Prince, describing him as an understanding, empathetic and warm man. Kate also spoke about Heads Together envision for the coming weeks, ahead of the marathon 23rd April:

"What Heads Together is proposing is that in the weeks leading up to the marathon, our campaign will showcase people from all walks of life, talking about the life-changing conversations that have helped them with their mental health challenges".