LONDON (Reuters) - When a teenage Michael Phelps started splashing the girls at the end of a particularly tough training session, his coach Bob Bowman tried to discipline him.
"I said, 'you should be very tired, that's the hardest practice you've ever done,'" the coach recalled.
"I'll never forget. He looked me straight in the eye and said 'I don't get tired.' So I made that my life goal, to see if I could accomplish that."
Twenty-two Olympic medals later - including 18 golds - the greatest Olympian finally retired from competitive swimming at last week's London Olympics. Much more than raw energy drove the boy from Baltimore through race after lung-bursting race.
To understand Michael Phelps, you also have to talk to the man with a psychology degree who trained him, who knew exactly when and how to rile him, who drove Phelps almost to the point of rebellion.
Bowman, 47, is quietly spoken, white haired and bespectacled. He has none of the air of the poolside bully. But that's one of the roles he played.
"I've always tried to find ways to give him adversity in either meets or practice and have him overcome it," Bowman has said.
"The higher the level of pressure, the better Michael performs. As expectations rise, he becomes more relaxed... That's what makes him the greatest."
Phelps didn't take easily to swimming as a five-year-old.
"I was afraid of the water at first, I didn't want to put my face under," he said. "I just didn't like the feeling."
Born on June 30 1985, Michael was the youngest of three children. His father, Fred, had been a college football player who once tried out for the Washington Redskins. His mother, Debbie, was a schoolteacher who became a Middle School principal.
In elementary school, Phelps was diagnosed with attention-deficit hyperactivity disorder - he used to take medication on school days. He was bullied. His parents divorced when he was nine and his mother, who raised him, encouraged him to follow his accomplished older sisters into swimming.
It gave him a focus. By age 10, he broke his first national age record. He started training with Bowman the year after that.
Bowman says that from the moment he met Phelps he knew he had discovered the once-in-a-lifetime swimmer every coach dreams about.
"When I went home that night I couldn't sleep I was so excited, but of course I didn't tell him that."
Phelps was the youngest and would start practice at the back of the line of swimmers doing laps in the pool, Bowman recalled. But by the end of the training session, "I saw a little cap moving up forward to the front of the line with each repeat swim.
"It was so remarkable, I'd never seen anything like it."
He asked the boy to pick his three favorite races, nominate the times he wanted to achieve for each and make that his goal for the year.
"He was just 11 but six months later he swam those exact times, to the one-hundredth of a second," Bowman said. "I don't know how that's possible, but it's true."
The excitement was tempered by the realization that Bowman himself needed to improve his own coaching skills and come up with new ways to get the best out of the young athlete.
"I wanted to impress them and think I was tough, I gave them an extremely difficult training program." He started by increasing the workload. Phelps coped with every task.
When Phelps was 13, Bowman made him swim 21 races in three days.
For good measure, when the boy was swimming at one of his first national junior meets, he let him race without goggles.
Bowman noticed Phelps had left his goggles behind just before he walked out to the blocks. To swim without them would make his eyes sting red with chlorine or he would have to close them.
"I could have taken the goggles to him but I decided to keep them and see what he could do," Bowman said. Phelps won the race without the goggles, and though Bowman didn't know it at the time, it was to prove a masterstroke.
In Beijing in 2008, Phelps dived in for the 200 meters butterfly and his goggles filled with water. Unperturbed, he swam blind to a new world record.
Outside the pool, Phelps found an idol and the lure of fame. He was 15 when he became the youngest American male swimmer to make the Olympics since 1932 and when he arrived in Sydney for the 2000 Games, he entered a new world.
The first billboard he saw was of a swimmer. Swimming was on the front and back of every newspaper and the lead item on the TV news. It was the biggest show in town, and the star attraction was a teenage boy, but it wasn't Michael Phelps.
Shy and unassuming, Ian Thorpe had a seductive technique that made swimming look easy. Propelled by huge feet and hands the size of saucers, he won his first title when he was just 15 and set about demolishing world records in his two pet events, the 200 and 400 meters freestyle.
Thorpe won three gold and two silver medals in Sydney: Phelps hardly created a ripple and the disappointment at not winning a medal made him redouble his efforts. "I was hard on myself for not achieving it."
It was at the 2003 world championships in Barcelona that Phelps first replaced Thorpe as the king of the pool, winning four gold medals and setting five individual world records.
Still just 19 at the time, he had grown into his 1.93 meter (6ft 4 in) frame. Most people have an armspan equivalent to their height, but Phelps' reach is about three inches longer. His size 14 feet work like paddles and his double-jointed elbows, knees and ankles add flexibility and propulsion.
Working with Phelps after his parents' divorce, Bowman taught him not only discipline but also how to do up his tie before his first school dance, and how to drive.
Like father and son, the two argued constantly, even about music: Phelps is into hip-hop where Bowman favors orchestral pieces.
Bowman spends his spare time playing the piano, and breeding and racing horses. He once considered naming a horse after Phelps, he said, but on consideration decided that would be unwise - it would put too much pressure on the horse.
Sometimes he would play mind games with the young swimmer. Before a Beijing race, Phelps recalled, "Bob said to me it would be good if I lost. When he said that, I was fired up, I said 'I'm going to go for it.'"
And Bowman has goaded Phelps about rivals. In Beijing, he made sure the swimmer knew before a race about comments made by Serbia's Milorad Cavic, the Serbian who had said it would be bad for the sport if the American won eight golds in Beijing.
"I brought it up on the way to breakfast," Bowman said. "I wasn't going to, but I threw it out there."
Phelps clawed his way from a seemingly hopeless position to beat Cavic by one-hundredth of a second, winning his seventh gold and equaling the record set by Mark Spitz at Munich in 1972. With the help of a relay he went on to make it eight - a feat that brought him a $1 million prize from his sponsors.
"I wanted to do something no one else had done in swimming," said Phelps. "I'd rather be the first Michael Phelps than the next somebody else."
He spent the $1 million prize establishing a foundation to help disadvantaged children learn how to swim.
The day after his Beijing triumph, Phelps' head was spinning. "I don't know where I'm going or what I'm doing, I'm just along for the ride," he said.
Within minutes of receiving his gold medal, he had taken a call from U.S. President George W. Bush, who told him to make sure he hugged his mother. He received nearly 5,000 messages on his BlackBerry but didn't have the energy to read them all.
He spent the night having a quiet dinner with his mother and sisters. He ordered a cheeseburger and did not drink a drop of champagne or any alcohol.
Phelps realized that now the only way was down. A whole new direction.
He declared he would compete at London but ruled out an attempt at nine gold medals or even eight. He started to skip training and put on weight, making regular trips to Las Vegas and hanging out with college pals.
He partied too hard. The newspapers published photographs of him inhaling from a glass pipe used for smoking marijuana. Phelps apologized and was suspended for three months.
His medal tallies were getting lower. At the 2011 world championships, he was beaten by countryman Ryan Lochte in two races.
The losses proved the jolt he needed. For the first time since Beijing, he could see a challenge.
Each morning, he got up before dawn, packed his swimming trunks, goggles and towel and headed to the pool. He returned each afternoon, logging up to 70 km (43 miles) a week in laps.
He was not the same swimmer as in Beijing, but London was to be an everlasting reminder of his determination.
After a poor start, he fought back. He anchored America to gold in the 200 meters freestyle relay final to claim his 19th Olympic medal, overtaking Soviet gymnast Larisa Latynina's overall total of 18, which had stood unchallenged for nearly half a century.
He was not quite finished. He beat Lochte in the 200 meters individual medley to become the first male swimmer to win the same individual event at three Olympics. Then, through sheer force of will, he won the 100 butterfly after lying seventh at the turn.
He finished his career with 18 gold medals, six from Athens, eight from Beijing and four from London. When he stood on the podium for the last time, his eyes filled with tears.
"I said 'I love you' first, we hugged," said Bowman. He said 'We did it,', we smiled and I said 'yes we did' and that was it. He's done all he can do here."
Editing by Sara Ledwith and Richard Woods