Kate Middleton just revealed a surprising thing that her and Prince William's only daughter, Princess Charlotte, enjoys.

On Wednesday, the Duchess of Cambridge visited one of her patronages, the Anna Freud National Centre for Children and Families (AFNCCF), a children's mental health charity, for the opening of its new Kantor Centre of Excellence in London's King's Cross. During her royal engagement, she stopped by the center's interconnecting Pears Family School, where she got to talk to a little girl.

During her conversation with the young girl, Middleton talked about slime, which is apparently one of Princess Charlotte's favorite pastimes, Hello! reported.

"Charlotte, my daughter, she dropped hers on the floor and it was pink and ended and brown and covered in so much mud. It ends up all gooey, doesn't it?" Duchess Kate told her.

The young girl's grandmother later opened up about her granddaughter's interactions with Prince William's wife, saying that their conversation was very "funny."

"The Duchess was lovely with her. It was quite funny that they were talking about slime, of all things," the grandmother said.

Kate Middleton and Princess Charlotte
Pictured: Middleton and Princess Charlotte after attending Prince Harry and Meghan Markle’s wedding at St George's Chapel, Windsor Castle, in Windsor, on May 19, 2018. Getty Images/Jane Barlow

This came a day before Princess Charlotte turned 4 years old on May 2. To celebrate the young princess' birthday, Middleton and Prince William released new portraits of their only daughter via Kensington Palace's Twitter page.

In the photos taken by Middleton herself, Princess Charlotte played and posed on the grass at Kensington Palace and the Cambridge family's home in Norfolk. One snap showed Prince George's younger sister the lovely Betsy Dress from high-street store Trotters that reportedly costs £70, paired with £26 navy Hampton Canvas Plum Shoes from the same brand, according to Daily Mail.

The dress is a recycled piece that she previously wore when she and brother Prince George visited their younger sibling Prince Louis at the hospital following his birth last year.

The two other photos showed Princess Charlotte wearing a £50 Yarrow tartan plaid skirt from Amaia. This is another recycled piece, one she previously donned for the Burnham Horse Trials last month. The young royal paired the skirt with a navy blue roll neck top, light gray button-up cardigan and tights.

Check out Princess Charlotte's latest photos below!