Diem Smallenbroek used to look at Harry, the rubber mannequin with acrylic hair and a sterile smile in her CPR class, and wonder what she'd do if a real person needed help. "In class, we'd all say that if somebody ever drowned we'd just panic, we wouldn't know what to do."

But when Diem's neighbor came rushing to her front door Saturday, screaming that her 2-year-old son had fallen into their swimming pool, it all "just clicked in."

Joshua, the youngest son of Michael and Brandi Aisenberg, was listed yesterday in serious but stable condition at Fairfax Hospital. The parents said doctors had removed a respirator and the blond boy was breathing on his own.

But it was the small puffs of air that Diem injected into Joshua's water-clogged lungs that kept him alive for the critical minutes that preceded the arrival of paramedics to his Reston home at 11828 Blue Spruce Rd.

Diem, who is 15, was baby-sitting her 5-year-old half-sister, Krisy, when Brandi Aisenberg came to her house, screaming "Help me! Help me! The baby's dead! He's dead!"

Diem had told her neighors that she knew CPR, and so when Joshua, who minutes earlier had been playing in a sand pile, was found floating face-down in the pool, Brandi Aisenberg knew whom to turn to for help.

The hardest part, Diem says, was concentrating on the baby, wet and helpless on the Aisenbergs' kitchen floor. "I just had to block everything else out," she says.

Diem says she tilted the baby's head, felt his pulse below his chin, put her mouth over his nose and mouth, and was careful to control the amount of air she fed into his tiny lungs. "Two little breaths every five seconds," Diem recalled yesterday. But no, that's wrong. And she corrects herself. "It was one little breath every two or three seconds."

Saturday she measured her breath instinctively. "When you've had CPR," said Joshua's father, "you can just click in. And Diem just clicked in yesterday."

Diem had been baby-sitting for Joshua since the Aisenbergs had moved next door in August. But Diem says the little boy is shy and she had just begun to know him. When Joshua was suddenly thrust into her arms, "I felt close to him . . . . I just couldn't let him go."

So, though his body was cold, his lips and ears were blue and his mouth was spilling food and water, "I tried to reach out to him."

She tried to forget she had a little human being in her arms. "But it was hard, because with Harry the mannequin you can't feel anything. He's not warm and and he's not cold," she says.

And she tried to forget that so much was at stake. "Working with mannequins, they have no life, and if you make a mistake you can go through the routine again," she said. "But with a real person, you're not allowed any mistakes."

A day after Joshua's near-drowning, Diem can't help talking about what she wants to tell students who are learning CPR at South Lakes High School, where she is a sophomore. "I would like to talk to health classes and tell the students to take them seriously. Because the kids in my class were not taking it seriously. They thought it was a big joke."

But Diem knows otherwise: "In one moment, you see a happy boy playing with his mother. And the next moment, he's depending on you for his life.