WASHINGTON (CBS DC) — Stephen Hawking’s son, Tim, admitted to placing swear words in his famous physicist father’s voice box as a childhood prank.

Tim Hawking, 36, detailed how he converted his father’s wheelchair into a go-kart as a child and discussed his father’s loss of natural speech as a result of his motor neurone disease. The University of Cambridge cosmologist and “A Brief History of Time” author suffers from a rare early-onset form of amyotrophic lateral sclerosis (ALS), also called Lou Gehrig’s disease.

Stephen Hawking, 73, communicates using a single cheek muscle attached to a speech-generating voice box.

The legendary physicist and other members of his family spoke with comedian and self-described physics fanboy Dara O Briain for a BBC1 documentary. Tim is the third of Hawking’s children with first wife, Jane Wilde. Hawking’s sister Lucy, 45, was also interviewed for the BBC1 documentary and both reflected on their unique childhood experiences growing up with Hawking.

Tim, who currently works in marketing for the toy company Lego, said at one point he placed curse words into his father’s speech machine as a prank, would use his father’s wheelchair as a go-kart and would often have intense board game battles with his “hugely competitive” father – particularly long games of chess.

In the documentary interview, Tim discussed the emotional impact of finally being able to communicate with his father after he received his speech device.

Hawking lost his natural voice completely after a tracheotomy following a bout with pneumonia.

“My dad was able to speak with his own, natural voice for those first years, but it was incredibly difficult to understand what he was saying – particularly for me at such a young age,” Tim says in the documentary. “As a three-year-old, I had no understanding of what he was saying. I didn’t really have any communication with him for the first five years of my life.

“It was only when he got his voice synthesizer that I was actually able to start having conversations with him,” continued Tim. “It was somewhat ironic that Dad losing his voice was actually the start of us being able to form a relationship.”

Hawking was awarded the National Medal of Freedom and dubbed an “agent of change” at the White House by President Barack Obama in 2009.

Benjamin Fearnow