Kate Middleton has perfect one-liner after Prince William promises he'll run a marathon

Duchess Kate and Prince William gave us more #CoupleGoals after showing they're not afraid to make fun of each other.

The royal couple and Prince Harry attended an event on Tuesday to highlight their mental health charity Heads Together -- and it'll be certainly an affair to remember in more ways than one.

"I chatted to Prince William, and he has promised that he will run a marathon in Kenya sometime," Good Morning Britain's Sean Fletcher told reporters.

In good fun, his wife responded with a zinger: "I'll believe it when I see it."

Ouch, that one's gotta hurt!

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And while running 26.2 miles is a grueling feat even for the most impressive athlete, it'll probably be no sweat for Duchess Kate's husband! The 34-year-old royal is actually quite a sportsman after years of playing polo. He still plays the game, often appearing in charity matches with his little brother.

Duchess Kate, Prince William and Prince Harry joined forces in an inspirational speech to highlight their goal of normalizing the conversation surrounding mental health at the charity event in London.

"Mental health matters to each and every one of us. It matters just as much as our physical health," the Duke of Cambridge said.

His wife also agreed. "We have seen that two heads are better than one when dealing with a mental health problem," and added, "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."

Of course, Duchess Kate looked stunning as ever at the event, wearing a long-sleeved floral frock that hit just above her knees. She wore her hair down in a long and voluminous hairstyle.