It’s probably pretty hard to keep a straight face when you’re standing between two handsome princes, but you’d think Kate Middleton would be used to it. In a new video for Heads Together, a mental health initiative, the Duchess of Cambridge can’t seem to keep a straight face.

At the end of the promotional clip, a bit of the trio’s photo shoot is shown. While William and Harry agree to wear serious faces, Kate can’t help but break into a big grin every time. Watch the video below:

Though she had trouble staying serious for the photos, the mother of George and Charlotte managed to keep it together when she had to deliver her message. “Mental health is just as important as physical health,” Kate tells viewers.

The duchess, 31, along with her husband, 33, and brother-in-law, 31, are supporting Heads Together as the charity of the year for the 2017 London Marathon, according to the official Kensington Palace Twitter account. The initiative will partner with several philanthropic organizations, including Best Beginnings, CALM and YoungMinds, to help erase the stigma surrounding mental illness.

“Too often, people feel afraid to admit that they are struggling with their mental health. This fear of prejudice and judgement stops people from getting help and can destroy families and end lives,” reads a letter signed by William, Kate and Harry on the Heads Together website. “Heads Together wants to help people feel much more comfortable with their everyday mental wellbeing and have the practical tools to support their friends and family.”

The launch of Heads Together is the latest in a string of publicity events for the Duke and Duchess of Cambridge. They recently returned from an official visit to India and then hosted a dinner for President Barack Obama and first lady Michelle Obama Friday night.

The mental health initiative, however, is an issue particularly close to Prince William’s heart. People reports he had to help several victims of suicide attempts while working with the East Anglian Air Ambulance, and that prompted him to focus on mental health efforts.