  The evolution of low-mass stars into red giants is still poorly understood.
During this evolution the core of the star contracts and, simultaneously, the
envelope expands -- a process known as the `mirror'. Additionally, there is a
short phase where the trend for increasing luminosity is reversed. This is
known as the red-giant-branch bump. We explore the underlying physical reasons
for these two phenomena by considering the specific entropy distribution in the
star and its temporal changes. We find that between the luminosity maximum and
luminosity minimum of the bump there is no mirror present and the star is fully
contracting. The contraction is halted and the star regains its mirror when the
hydrogen-burning shell reaches the mean molecular weight discontinuity. This
marks the luminosity minimum of the bump.
