The software entity is constantly subject to pressures for change. Of course, so are buildings, cars, computers. But manufactured things are infrequently

The software entity is constantly subject to pressures for change. Of course, so are buildings, cars, computers. But manufactured things are infrequently changed after manufacture; they are superseded by later models, or essential changes are incorporated into copies of the same basic design. Callbacks of automobiles are ready quite infrequent; field changes of computers somewhat less so. Both are much less frequent than modifications to fielded software. In part, this is so because the software of a system embodies its function, and the function is the part that most feels the pressures of change. In part it is because software can be changed more easily--it is pure thought- stuff, infinitely malleable. - Fred Brooks, No Silver Bullet