  With the rapidly growing number of cosmic microwave background measurements
on various scales, there is real hope that the number of acceptable models for
structure formation will be limited to a very few in the near future. Yet any
given model can always be saved by introducing and tuning extraneous free
parameters. To better understand this question of ``nature versus nurture'' for
temperature fluctuations, it is useful to know not only the general features of
anisotropy predictions but also their causes. Extracting the physical content
of our other works, we present here a {\it simple} account of cosmic microwave
background anisotropies on all scales. In particular, we show that analytic
approximations can trace the structure of the so-called ``Doppler peaks,''
which arise due to the {\it adiabatic} oscillations in the photon-baryon fluid.
We also show how the finite thickness of the last scattering surface and the
Silk damping mechanism can be described in a unified way by photon diffusion.
In order to present a specific example, we focus on comparing the primordial
isocurvature baryon (PIB) model with the standard cold dark matter model (CDM).
In particular, we explain why PIB generically predicts larger {\it
non}-oscillatory anisotropies from the 1$^\circ$ to 10$^\circ$ scale which may
already be in conflict with experiments.
