Kate Middleton likes to keep Prince William on his toes!
The Duchess of Cambridge hilariously shut down her husband's marathon dreams during an event for the mental health charity, Heads Together, in London, England, on Tuesday.
RELATED: Kate Middleton Visits With Local Families With Mental Health Charity After Celebrating Her 35th Birthday
"I chatted to Prince William, and he has promised that he will run a marathon in Kenya sometime," Sean Fletcher, of Good Morning Britain, revealed to the press, before sharing Middleton’s joking response: "I'll believe it when I see it."
Middleton's joke provided a lighthearted moment following a serious discussion on mental health. The royals couple and Prince Harry shared a joint speech at the event.
"Talking to someone else is a positive and confident step to take -- but for too long it has been a case of 'Keep Quiet and Carry On'," William told the press during the address. "As a result, too many people have suffered in silence for too long, and the effects of this can be devastating."
READ: Kate Middleton Goes Casual For Cub Scout Pack Meeting -- See the Pics!
Kate pointed out that "two heads are better than one" when it comes to battling mental health ailments.
"William, Harry and I have been very privileged to witness in the course of our work countless examples of simple conversations that have changed lives, which were the first step on a path to recovery," she said.
Harry added that seeking professional help isn’t the only way to battle mental health issues.
"We can all help each other," he noted. "You don't need any qualifications to help your mate, simply to listen to what they have to say."
WATCH: Kate Middleton Dazzles at Diplomatic Reception in Princess Diana's Favorite Tiara -- See the