BOUNTIFUL — Callahan Williams kneels beside the pad where daughter Eliza, 3, has been carefully positioned for comfort. He puts his face inches from hers and makes a loud popping sound. Her breathing quickens briefly, and her lip twitches.

They're playing.

The swinging games and tickling tag, the peek-a-boo and clapping are gone, banished by a degenerative neurological illness that, in the space of a few months, has taken a sociable, rambunctious toddler backward through time.

They loved to blow bubbles, and he still does it, giving a sunny commentary as the bubbles wend their way to her forehead or hand, where she can feel cool moisture before they break. Her little tongue vibrates slightly on her lower lip.

He thinks — hopes — that she's laughing.

Eliza was diagnosed last Sept. 11 with metachromatic leukodystrophy, and by Christmas she had lost the ability to walk, to talk, to sit up, to move her hands with purpose. The little girl who loved food can no longer swallow, so she "eats" through a tube that transports Pediasure directly into her stomach.

For a while, she could say "uh huh," but that's gone now. Her vocalizations now are little grunts and squeals, and they're rare enough to spark excitement from her mom, Nikki.

In the past six weeks, Eliza has lost head and neck control. She can no longer see.

It's not all gone, though. Nikki Williams says there's something bittersweet about what has been left behind. Eliza can still think; experts say the disease does not diminish cognitive ability, just the ability to use it in a meaningful way.

Dressed in pinks and yellows, her favorite colors, eyes half-closed as she reclines against a cushion, she looks like one of the little princesses she was starting to love when the disease changed everything.

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Nikki and Callahan Williams, both 29, are Utah natives who met at the University of Utah. They married, and he started his own software business. When Eliza was born — on 03/04/05 and "how cool is that?" her mom adds — everything seemed perfect.

Nikki used to teach special education, and she says she always watched Eliza hit milestones. Until age 2 she was, if anything, ahead of her peers, her mom says.

She adored animals, especially the odd ones with unusual names: dolphins and otters and peacocks and camels. By the time she was 18 months old, she could name them all and spent happy hours playing with her Little People Barn and Noah's Ark. An especially good day involved going to the zoo, then coming home to watch movies about animals.

"Moo-oooo," she'd say. "Quack, quack."