Stephen Hawking was one of the world's most famous physicists, but he was also a loving dad, with three children born between 1967 and 1979.

One of those kids, Tim Hawking reminisced in a 2015 BBC documentary about his father's fight with amyotrophic lateral sclerosis (ALS) which slowly robbed him of his ability to verbally communicate. The elder Hawking passed away March 14, 2018 at the age of 76.

"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 told BBC host Dara O’Briain. "As a 3-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."

The breakthrough that brought them all closer together happened when his father received a computer-driven speech program called "Equalizer."

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

Playing pranks on his dad

Tim Hawking says he bonded with his father bonded over games like chess. (Photo: Snapshot from BBC video)

Tim revealed that as a child he would play pranks on his father — including adding swear words to his speech program and using his specialized wheelchair as a go-kart. The pair also bonded over board games — and they were fiercely competitive.

"There was no compassion at all," Tim shared. "My father is hugely competitive, and he certainly wasn’t the easiest opponent at any game, particularly chess."

As for the Academy Award-winning drama, "The Theory of Everything," about Hawking's younger years, Tim said watching it was incredibly emotional.

"I’ve never known my dad as an able-bodied person," he said. "So to actually see him as a young man — that was one of the really lovely things about the film for me."

You can watch the BBC documentary in its entirety in the video below.