The triathlon is the ultimate test of endurance. Athletes train for years, mastering three different skills - swimming, cycling and running. They must push their body to the absolute limit.
Such a challenge is too much for many grown men and women. But it hasn't put off Nick Yip Tak-long, aged 10, and his brother Sean Yip Tsz-hin, eight. They've been training for triathlons since they were just six years old.
"Both Sean and I started learning the three sports separately," says Nick, a Primary Five student at Po Leung Kuk Camões Tan Siu Lin Primary School. "At first, it was running for me and swimming for Sean.
"Then we gradually discovered our potential, so we began to train in triathlon. We both prefer running and cycling, because we can enjoy the scenery along the way."
Despite the nice scenery, training for a triathlon is no walk in the park. Nick spends around 20 hours a week training, with eight of those hours focused on swimming. Sean trains up to 12 hours a week.
"I think the most difficult part is the weather," says Sean, a Primary Three student at Diocesan Preparatory School. "When we swim, sometimes the weather is hot and sometimes it is cold, but we still need to bear it, train and compete."
Perhaps the toughest part of a triathlon is switching between the different disciplines.
"When I finish swimming I often feel dizzy and my legs lack energy," says Nick. "But I can't slow down because it is important to use every second to grab my bike and carry on."
For a junior triathlon, the boys must swim between 100m and 400m, cycle between 4km and 12km, and run for between 800m and 4km. But Nick is already dreaming of taking on an Olympic triathlon: a 1,500m swim, a 40km cycle and a 10km run.
The brothers' mum, Winky Yip, says that the training is really helping her sons. "During the competition, they have to be independent," she says. "We are happy to see that they are growing up as strong individuals."
Nick and Sean's parents take them to competitions in Hong Kong and abroad. In August, the brothers competed in the Singapore International Triathlon. Nick finished first in the boys' 10- to 11-year-old category, while Sean placed third in the eight to nine-year-old category.
And a friendly rivalry between the brothers is helping these "boys of steel" become even stronger. Nick is hoping to represent Hong Kong at the Olympics in the future. And Sean has his own dreams, too.
"I don't have a concrete goal," he says. "For now I will train hard and hopefully be as good as my brother."