SAN FRANCISCO -- Fish gotta swim and birds gotta fly -- even if they're just animated creatures. The computer animators at Pixar Studios spent three years making sure their little charges in Finding Nemo could convince audiences they were capable of movement, flight and emotion.

Finding Nemo is the fifth feature-length movie from Pixar, coming on the heels of Toy Story, A Bug's Life, Toy Story 2 and Monsters, Inc.

The fish tale hauled in $70 million US for the best opening weekend ever for an animated flick. Finding Nemo sank the previous record of $62 million for Monsters, Inc.

"Our objective at Pizar is to make our characters believable, not realistic. We're not asking people to believe these are real fish, real bugs or real toys but we do want them to believe in them," says John Lassiter, who founded Pixar and is the executive producer of Finding Nemo.

When Pixar created the short film Knick Knack in 1989, Lassiter was the only animator at the company and he operated out of a warehouse in San Francisco.

"Back then, we were essentially a computer company with an animation department. Toy Story turned us into an animation studio.

"With A Bug's Life we vowed to have an animated short accompany each feature film. This year, with Finding Nemo, we're rereleasing Knick Knack."

In November 1999, Pixar moved into a multimillion-dollar facility in Emeryville on the outskirts of San Francisco. The company employs 48 full-time animators and, at the height of production, there were as many as 180 animators working on Finding Nemo.

Pixar is at the forefront of computer animation but its films still take three to five years to complete.

"It takes 96 frames of film to create one second of movie and at our peak we're lucky to complete five seconds a week.

"This is why the films are expensive and time-consuming but we feel it's all up there on the screen. Our goal now is to release a new feature every year.

"We're able to manage one every 18 months but we hope to start the one-a-year by 2006," says Lassiter.

Already in production are Incredibles for the fall of 2004 and Cars for fall "Incredibles is a superhero movie from Brad Bird, who created The Iron Giant. It's going to be Pixar's first movie with humans as the major characters.

"It's very challenging for computer animation because of the clothing, skin and hair. Creating a Buzz Lightyear is infinitely easier to do."

Lassiter himself is directing Cars, whose characters are -- not surprisingly -- automobiles.

"Our films have to translate easily beyond American English because more than half our box office now comes from international markets.

"This is something our writers have to keep in mind. It's a major priority. We've already dubbed Finding Nemo into 36 languages."

Lassiter is proud of the look of Finding Nemo.

"With Nemo we pushed computer animation to a new level, but our model was Disney's Bambi."

"Those animators were able to achieve such believable movement and motor skills and achieved such emotional expression. Bambi also had a magnificent impressionist style.

"We thought of Finding Nemo as Bambi under water."