  Viscoelastic silicone rubber (VSR) is a remarkable shape-memory solid. The
material's polymer network retains a memory of its shape history, so its
current and future shapes depend strikingly on its past shapes. Although VSR's
memory fades gradually and it has a permanent (cured-in) shape to which it will
eventually return when left alone, VSR can be taught new shapes and retain them
for significant lengths of time.
  To examine VSR's ability to learn, remember, and recover shapes, this work
focuses on a simple experiment. A VSR that has relaxed into its permanent shape
is suddenly compressed to about 80% of its original height. After a specific
period of compression, the VSR is released and allowed to return to its
permanent shape. Having learned a new shape during the compression period,
however, the VSR is reluctant to return and takes seconds, minutes, or hours to
do so, depending on how long it was compressed.
  In addition to observing these behaviors experimentally in VSR, we show that
those behaviors are well-described by the Fractional Zener model. That model
involves time derivatives of non-integer order and those generalized time
derivatives have memory. Both VSR and the Fractional Zener model describing it
are acutely aware of the past.
  The model's mathematical machinery make it possible to design VSR behaviors
based on physical parameters, although some of the model's relationships are
not yet known in closed form. VSR's existence as a practical material means
that devices can be designed and produced that use a memory of past shapes to
do things that would otherwise be difficult or impossible to make.
