How to Hold Your Breath

a short, short story. Needs some filling in. All suggestions appreciated.

    This spot, at the edge of the lake, had been her Eden this summer.  She spent nearly everyday there, even when her sisters did not want to go,  swinging from the rope swing, catching frogs, or just reading a book under one of its trees.  The canopy of branches protected her from the sun and filtered the light so that certain parts of the grass shone bright green. 
    “Even thinking uses oxygen,” Robert warned her.  Kate stared at the stopwatch in her hand, wondering how it was possible to completely stop thought for one minute and twenty seconds.  She picked at the grass waiting for Robert to come up from the water.  The sun highlighted the whiteness of her legs and the reflection of the stopwatch‘s face bounced over her as she shifted.  She was trying not to think, but she became nervous.  Shouldn’t he be up by now?  She was about to jump in after him when he swung his head out of the water.  He parted the brown, murky lake as he swam to shore.
    “Beat that,” he grinned, walking toward her.
    Kate did not know if she could just stop herself from thinking.  “Ok.  Let me practice first.”
    She imitated what Robert had done.  She took several short breaths, then a final gulp of air and closed her eyes.  No thinking, she reminded herself.
    Her thirteen year old body was not yet as developed as her sisters’, or even some of her classmates’.  Her frame was made up of thin hard edges, her skin taught over the bone.  She had not expected to be there alone with Robert that day.  She wore blue jean cutoffs and a white ribbed tank top.  She could feel the wet shirt hugging her flat chest.  She realized then that Robert could probably see every bit of her undeveloped self.  She panicked, her mind raced, she needed oxygen.  She immediately opened her eyes to make sure he wasn’t looking.
    “Five seconds, Kate.  That must be a new world record.”
    “Shut up,” she said trying to sound tough.
    “Good thing I’ve never tried to kiss you, you’d be gasping for air before I could slip you the tongue.”
    “No I wouldn’t, just ask,” she hesitated, “around.” Kate’s arms were wrapped tight around her own chest.  She quickly took in a deep breath and closed her eyes.  She concentrated hard on the blackness, vaguely hoping she could disappear into it. 
    He leaned down and kissed her.  The slight pressure caused her to take a step back, and her back dug into the bark of the tree trunk behind her.  Her eyes sprang open and she blew out her breath in his face.  Robert was grinning wide.  Kate noticed the droplets of water forming on his tanned chest. 
    Instinctively she wiped her mouth with the back of her hand.  “What’d you do that for?”
    “Well, I thought you were all experienced and everything.”
    “How am I supposed to not think if your going to…to do that.”
    “I was making it more challenging, seeing if you could survive under pressure.”
    Her mind was flooded with thoughts, but she could not pick even one that sounded right to put into words.  She pushed him back toward the water, “go show me how it’s done again.  Let me practice up here by myself.”
    “You can’t very well practice kissing by yourself, Kate.”
    “You know what I mean.  See if you can hold you breath for two minutes, or better yet try to stay down there forever.”
    Robert jumped back into the water.  His splash showered the bottoms of her legs.  She took a deep breath and tried to clear her mind, but it was too late.  She was flying with excitement.  She ran and flung herself into the lake.  She let the water submerge her.  She could not even see the bottom of her feet through the dirty lake, but she now possessed the knowledge of a whole new world.  She followed Robert as he swam to the lake’s center.