While Charles appeared delighted at his gag, the Queen was the model of restraint and remained expressionless.
Luxembourg's Xavier Bettel, Belgium's leader Charles Michel and Norway's prime minister Erna Solberg also appeared amused, which may have been some comfort to the future King.
It’s been a busy week for the Queen and the royal family, who welcomed Donald Trump for his state visit on Monday, which concluded with the D-Day commemoration.