What's Underwater, Stays Underwater

Life for Jenny was normal when she a kid. She wasn't really popular, and she didn't really love school, but life was normal. Then, she turned ten. The biggest day of her life so far. Double digits. Um, can you say EXCITING? Yeah, well that night, her family had a little pool party for Jenny in their backyard. They set out a cake, some mini hot dogs, and invited all of her friends. But, no matter how hard Jenny pleaded, begged, and cried, Millie, her little sister, also attended. (Well, it WAS her house, too.)
Surprisingly enough, annoying, five year old Millie didn't bother anyone at all. Jenny didn't even SEE her. After all of her friends went home, Jenny, was drying her hair with a towel. That's when it hit her, how come Millie was so quiet? She asked her parents if Millie was there, they said they saw her jump in the pool with her little floaties. Jenny, her mother and her father, all looked at each other, realizing, Where's Millie?
They ran upstairs downstairs, in every bathroom, even outside. Nowhere to be found. The parents sat down, about to call the police, when Jenny had the brilliant idea of turning the pool lights on. There she saw, with horror, Millie at the bottom of the pool.
She screamed.
Her parents came running out, asking what happened. And they also saw Millie at the bottom of the pool.
But what was she doing?
She was crisscross, and when she saw the family looking down at her, she waved. The parents motioned her to come up. She swam up, quickly, not even taking a breath of air.
"What were you doing? You had us so worried." Said her mother, near tears.
Millie simply replied, "Swimming."
Jenny said, "How long have you been down there?"
Millie smiled her toothy smile. "Since the party started."
The parents looked at each other, and the mother burst out crying.
Their father rushed them into the dining room, where they all sat down at the dining room table.
Mother doesn't usually let us sit at the dining room table in bathing suits, Jenny thought.
"My father had... had..." She started trembling and pointing at Millie. Jenny was scared, although Millie acted normal.
Their dad, completely calm, sighed, and put his arm around his wife. She relaxed a little.
Looking at both Jenny and Millie, he said, "You grandfather, well, he had a power. It was that he could breathe underwater and on land."
Millie snapped her fingers. "I was breathing under water!" She said smiling.
No one smiled back.
That was the day that Jenny's life changed.