Prince William has told a group of young environmental entrepreneurs that their influence on older generations could create "momentum" in the battle for a greener planet.
The Duke of Cambridge was speaking to seven young people who have been named 2020 Young Champions of the Earth by the UN Environment Programme (UNEP).
He told them they could also "be in the mix" for his £50m Earthshot Prize, described on its website as the "most prestigious environmental project in history".
Nzambi Matee, from Kenya, who manufactures sustainable building materials, told William it was difficult to change the attitudes of older people.
She added, however, that her grandmother had adopted sustainability without any fuss.
"If we can convince my grandmother not to use plastic bags, we can do anything," she quipped.
The duke agreed, saying: "If every young person educates their family on the environmental impact they are having, that in turn is making a difference, and changing the tide, and creating that momentum."
Others recognised include an engineer who turns plastic rubbish into paving stones, and someone who is fighting to save endangered salmon.
The winners will receive more than £7,000 in seed funding, plus tailored training to help scale up their ideas.
William said he was "hugely honoured" to speak to such "brilliant young people doing such fantastic things".
He launched his Earthshot Prize alongside his Royal Foundation in the autumn, and UNEP is a global alliance partner.
The aim is to find solutions, ideas and technologies that "repair the planet".
The duke told the group that Earthshot had been started to "bring hope and optimism back to the environment debate" and to "try and encourage change through hope and action rather than pessimism and despair".
He asked them: "Why do you think the optimism part, which I felt very strongly about, is so important?"
Lefteris Arapakis, from Greece, who co-founded an organisation that teaches sustainable fishing, said the pandemic had "changed everything".
He added: "I am from a family of fishermen and every year for the last 20 years we get less fish, my family has less and less of everything.
"Personally, I believe that we can make the change, because if we don't believe that, we can just give up. It's our only choice. Optimism is our main weapon against the climate crisis."