But when the first question, ''Who knows what Earth Hour is?'' received a unanimous raising of hands, the talk changed track.

''They are all over it, all over it,'' Mortlock said, turning the conversation to how the event had spread to more than 80 countries around the world.

The Wallabies captain and Brumbies outside centre is a celebrity ambassador for Earth Hour again this year and said he was surprised by how much local pupils knew about sustainability.

''It is amazing how many of these young kids are totally over all the issues to do with climate change and understand the whole sentiment about what Earth Hour is about,'' he said.

Mortlock, who has a science degree from Sydney University, is an advocate for Earth Hour and will be taking the message to South Africa during the Brumbies' tour, which coincides with the day. Many local schools, including Chapman Primary, will be taking part in Earth Hour this month.

Most will observe the hour a day early on Friday so the whole school can become involved, but pupils will also be encouraged to take part in the main event at home on Saturday, March 28.

Chapman Primary principal Anne Simpson said the school, an accredited waste-wise school, had a strong focus on sustainability.

Year 6 pupil Sarah Haider, 11, said she would observe the hour at home and at school.

''I think Earth Hour is important because we want to stop global warming because it is bad for our Earth,'' she said.

Year 4 pupil Robert Wilson, 9, said it was ''cool'' to meet Mortlock. ''I think Earth Hour is important because we are saving our world,'' he said.