Prince William: 'We'll both fall off a lot of times'
Princes William and Harry have set off on a 1,000-mile charity motorbike trek across South Africa.
The Royal pair are among 80 riders taking part in the eight-day rally from Port Edward to Port Elizabeth.
They aim to raise £250,000 for Prince Harry's charity Sentebale, Unicef and the Nelson Mandela Children's Fund.
Prince William told the BBC the trip was a "mixture of adventure and charity", with money being raised for "three absolutely brilliant charities".
"It's a great way of bringing all the three charities together and understanding the fact that Aids in Africa is still a major issue," he said.
Speaking before they set off, the 26-year-old prince said they had not done any specific training for what is billed as one of the most gruelling off-road bike routes.
"We both ride bikes at the moment on the road, but on-road biking is nothing compared to this.
"All the off-roads stuff is up hills, down slopes across rivers, and it's all rocky and hilly, it's going to be fun," he said.
His younger brother added: "It's not just a bimble across the countryside that's for sure, it's going to be very challenging and [we are] expecting to fall off many a time, every day."
The 24-year-old called the experience "hugely important".
Prince Harry's charity Sentebale, which helps disadvantaged children in Lesotho, especially Aids orphans, will receive around £100,000 from the event.
Money will also go to the other two organisations working in South Africa to help communities blighted by Aids or improve the prospects of youngsters.
The princes rarely get to spend time together due to separate commitments
The Enduro group, which is organising the trek, has raised nearly £1m over the past three years for the charitable causes favoured by the princes.
Prince Harry, an officer in the Household Cavalry regiment, said the princes were both looking forward to the challenge.
"We never really spend any time together - we've got separate jobs going on at the moment.
"But it's great fun - well I don't know yet, we'll have to tell you. We might argue, we might have a bit of fun."
Prince William said: "The pain of spending a week with my brother is well worth it."
Rachel Ward, from the Nelson Mandela children's fund, told BBC Five Live the teams were well prepared.
"We're delighted that they're drawing attention to the work that we're all doing for children in South Africa," she said.
The princes previously joined forces to stage the Concert for Diana last year in memory of their mother.