Camilla and Kate, and around 20 guests in the room laughed at Her Majesty’s joke.
The Queen then used a knife to cut a second slice of cake, remarking: “That looks very good”.
Peter Stewart, executive director of the Eden Project, said of Her Majesty’s visit: “It is like comets – they don’t come around very often.
“The important thing for me was there was every reason not to do it.
“The fact that they judged that with the eyes of the G7 that there was an opportunity – with the risk of missing trains and everything else – to be able to listen to some of the heartfelt stories of people here.
“It was absolutely priceless.”