  The long-time dynamics of quantum systems, typically, but not always, results
in a thermal steady state. The microscopic processes that lead to or circumvent
this fate are of interest, since everyday experience tells us that not all
spatial regions of a system heat up or cool down uniformly. This motivates the
question: under what conditions can one slow down or completely arrest
thermalization locally? Is it possible to construct realistic Hamiltonians and
initial states such that a local region is effectively insulated from the rest,
or acts like a barrier between two or more regions? We answer this in the
affirmative by outlining the conditions that govern the flow of energy and
entropy between subsystems. Using these ideas we provide a representative
example for how simple few-body states can be used to engineer a ``thermal
switch" between interacting regions.
