When Michael Phelps was a kid, his primary school teacher told his mother he would never amount to anything because he was unable to focus.

When Phelps won the first of his 14 Olympic gold medals, in Athens in 2004, he remembered those words as he stood on the podium and listened to the "Stars and Stripes".

Despite being diagnosed with Attention Deficit Hyperactivity Disorder (ADHD) at the age of nine, Phelps went to prove that teacher spectacularly wrong.

"He was a very energetic little guy, always all over the place, 'Why are we doing this? When are we doing this? What are we doing next?'," his mother Debbie told Reuters in an interview.

"Naughty isn't a word that I would use, he was playful, inventive," she added. "Definitely athletics channelled a lot of that energy."

By Michael's own account, naughty is a word some people might have used.

In his autobiography, he talks of being a "pool rat, running around, sneaking up behind people, stealing their snacks and goggles, tapping them on the shoulder and running away and just causing general havoc".

He was a handful at the dinner table too, because he always had to do something with his hands.

His mother, not surprisingly, loved the fact Phelps swam because she wanted him to burn as much energy as possible.

They are a close family, and Phelps hugged his mother and two sisters after winning his eighth gold of the Beijing Olympics. It is an achievement she says made her "very, very proud".

In between watching her son becoming perhaps the greatest Olympian ever, Debbie runs a school in Baltimore County, Maryland and has recently been signed up to a discussion group on Facebook giving advice to "ADHD Moms".

She says her son's extraordinary focus on swimming is common to many hyperactive children, unable to sit still for more than a few minutes at school but capable of devoting themselves entirely to something they love doing.

"You find children who have ADHD are very creative individuals, very determined, and they can focus, they can focus very intently on something that they love," she said.

Such is Michael's extraordinary obsession, he talks of "sleepswimming" as a child, waking the family up with a shout of "one, two, three... go". Even now, he says he literally dreams some races in advance, from start to finish, or visualises every "dive, glide, stroke, flip" as he is falling asleep.

Phelps' parents separated when he was young, his sister Whitney's successful swimming career derailed by a bad back and an eating disorder.

Phelps, though, makes it clear he drew enormous strength from the closeness of his family, and especially his mother's support.

He took medication to address the symptoms of ADHD, as do around 1 in 25 children in the United States, but only on schooldays and not at weekends or holidays. After a while, he weaned himself off even that, and has become an extraordinarily calm individual in the public eye.

And what about the schoolteacher who wrote him off? Debbie resists the temptation to crow, simply saying "she did not understand Michael at all".