Up in the air

Carl's house in "Up" is carried through the air by thousands of individually computer-animated balloons. Each balloon is interdependent on the others, meaning that if one bumps into another, the second moves, and so on. The physical simulator Pixar used for the modeling of the balloons was designed to build in random but realistic behavior by the balloons, and in one sequence, some of the balloons break free of the cluster. That was not intended by the filmmakers, but they kept the sequence in the movie because they liked it.