Prince William's son is ''obsessed'' with tractors.

The 37-year-old royal is pleased that his six-year-old son George, whom he has with his wife Duchess Catherine, loves playing on the big farming machines because it forces him to go outside and ''understand nature''.

Speaking in a new ITV documentary special, 'Prince Charles: Inside the Duchy of Cornwall', the Duke of Cambridge told farmer Mervyn Keeling as he visited the Duchy estate: ''I should have brought George today. He would be absolutely loving this... He's obsessed. My children are already, you know, playing on the tractors. It's so important to get outside, and have the children understand nature.''

And while George has a thing for tractors, his sister Princess Charlotte is ''pretty good with heat'' when her mother cooks curry for her husband and their children.

She said recently: ''It's so hard cooking curry with the family though.

''The children have a portion with no spice, [William's] is medium. And I quite like it hot. Charlotte is pretty good with heat.''

Two years ago, the couple - who also have 17-month-old son Prince Louis - admitted they love to relax with a takeaway curry while watching their favourite shows such as 'Game of Thrones' and 'Homeland'.

William said: ''We're actually quite keen on box sets. We like to watch box sets ... We've watched 'Homeland', we're big fans of 'Homeland'. 'Games of Thrones' we've watched as well. Seems like everyone has seen it... ''

Catherine, 37, added: ''Absolutely and curry -- definitely!''

But the greasy grub doesn't get delivered to their home.

William said: ''We go and pick it up, not ourselves.''

And asked what they wear during their chilled nights at home, he said: ''I've got my comfy clothes, yes.''