Kate Middleton took her early years project Stateside to share skills and ideas with experts at the world famous Harvard university - while Prince William marvelled at JFK.
The Princess of Wales came armed with a black notepad full of handwritten ideas and questions.
She spent half an hour with experts from The Center on the Developing Child at Harvard University.
Outside she went to both sides of the street to greet well-wishers and pose selfies and receive flowers.
The Prince of Wales, meanwhile, visited the John F. Kennedy Presidential Library and Museum, with Ambassador Caroline Kennedy, the former president's only living child.
Ambassador Kennedy, who was joined by her children Jack and Tatiana Schlossberg, accompanied William for a short tour around the library and museum.
It highlights JFK’s political legacy and his Moonshot programme that challenged America to put man on the moon.
William has said it is the key inspiration behind his Earthshot Prize.
During the visit, William was shown exhibits from the pivotal years of the Kennedy Administration in the 1960s, ahead of the moon landing in 1969.
This included a facsimile of JFK’s famous “We choose to go to the Moon” speech, which he delivered at Rice University, Houston, in September 1962, informing the public about his mission to land a man on the moon before 1970.
“Incredible,” William said, as he looked at the speech.
“What was he trying to do here?” he asked, pointing out JFK’s hand-written annotations on some sections of the speech.
Jack Schlossberg replied: “To a lot of people, he was trying to justify the investment that was going to be needed.”
The group also looked at a model of the Gemini space capsule which was presented to JFK in 1962.
“Did he have a big interest in space, was he quite scientific?” William asked them. “He was very much into the oceans,” said Ambassador Kennedy.
Towards the end of the tour, she also pointed out a silver-framed signed photograph of the late Queen which was presented to President and Mrs Kennedy during their visit to Buckingham Palace on June 5th, 1961.
After the visit, the Kennedys joined William for a private lunch and discussion with representatives from the Founding Partner organisations of the Earthshot Prize, before his meeting with President Joe Biden this afternoon.
Allison Morgan, 37, spent $1,000 on flights and a hotel to travel from Williamsburg, Virginia, to see the royal couple - and met Kate during her meet-and-greet outside Harvard.
She said: “She thanked us for coming out in the cold. It was worth every cent.
“I’m a huge fan of the royals. My mum was a fan of Diana and my grandmother a fan of the Queen.
"It is a family tradition.
“I have huge love for them and write and send cards.
“I do think Harry and Meghan could be more supportive to their brother and brother-in-law.
“They should be supporting Catherine and William when they are in Boston,"
Madeleine Arnold, 27, drove an hour with four-year-old daughter Harvest, to meet a “real-life Princess”.
Madeleine said: “It’s a once in a lifetime opportunity for my daughter to see a Princess here in Boston.
“One day when Catherine is Queen she will be able to say she saw her. So we had to try.
“I’ve told Harvest all about the Revolution and about England and she is very excited to meet a real-life Princess.”
Kate’s Royal Foundation Centre for Early Childhood has joined forces with the Harvard centre and she took part in a half an hour discussion to trade ideas and skills.
The Princess, wearing a houndstooth dress by Emilia Wickstead and earrings by Lenique Louis, told the experts it will “be the first of many”.
The Harvard Centre focuses on leveraging advances in science to inform policymakers.
They discussed how advances in science can be used to give every child a bright future.
Kate told them she had a lot to learn and wished to stay for longer - or even take a Harvard course.
During the meeting, Kate said: “There is so much amazing work going on and we need to start sharing the learning and science and what can be done better and be improved to help children in the trajectory into adult life.
She added: “It is frustrating because I’d like to be here for days.”
Kate told them: “My interest in early childhood started with the biggest societal challenges, homelessness and addiction, and the importance in how you build resilience with foundations and better mental health for people early on in life.”
Jack Shonkoff, Center Director, told their royal visitor: “We appreciate the work the foundation has been doing, it's a wonderful start. We want to share our science. We are at the frontiers of the science.”
He told her they wanted a global platform for the science studies for early years.
He compared their work to climate change science.
And described their work saying they study “stress and adversity get into the body and effect very early in life”.
Kate, who spent around half an hour with the six experts, smiled and said: “I need all day”.
After arriving earlier to cheers, Kate signed a visitors book placed next to an old book signed by the then-Prince of Wales when he visited in 1986 - marking the 350th anniversary of Harvard.
She said to her guests: “It’s so exciting to be here. I’d love to come and study but don’t know what to choose, as there are so many specialist courses available.
“Apparently they say it’s never too late, there is always time.
“It’s great to be here. So grateful for this partnership. I’m looking forward to learning more.
“There is a lot for me to know.”
For the last nine years, The Duchess of Cambridge has spent time looking into how experiences in early childhood are often the root cause of today’s hardest social challenges such as addiction, family breakdown, poor mental health, suicide and homelessness.
