Kate Middleton, Prince William, and Prince Harry take their charity work very seriously, but they also know how to have a little fun in the process. The royal foundation has teamed up with a number of mental health charities on an initiative called Heads Together, which aims to "change the national conversation on mental wellbeing" and will encourage people to run the 2017 Virgin Money London Marathon. The three are seen meeting with runners in a video for the project, and things quickly take a lighthearted turn. After Kate tells the camera that "mental health is just as important as physical health," the three put on headbands and pose for pictures. But after Harry suggests they "do a serious one," Kate can't keep a straight face. Watch the video above, then check out the moment that Prince George met President Obama in his bathrobe and slippers.