  We give six arguments that the Planck scale should be viewed as a fundamental
minimum or boundary for the classical concept of spacetime, beyond which
quantum effects cannot be neglected and the basic nature of spacetime must be
reconsidered. The arguments are elementary, heuristic, and plausible, and as
much as possible rely on only general principles of quantum theory and gravity
theory. The paper is primarily pedagogical, and its main goal is to give
physics students, non-specialists, engineers etc. an awareness and appreciation
of the Planck scale and the role it should play in present and future theories
of quantum spacetime and quantum gravity.
