  The {\em minimal unfolded region} (or the {\em heart}) of a bounded subset
$\Om$ in the Euclidean space is a subset of the convex hull of $\Om$ the
definition of which is based on reflections in hyperplanes. It was introduced
to restrict the location of the points that give extreme values of certain
functions, such as potentials whose kernels are monotone functions of the
distance, and solutions of differential equations to which Aleksandrov's
reflection principle can be applied. %the temperature of a heat conductor with
some initial-boundary condition, in which case the points are called the hot
spots. We show that the minimal unfolded regions of the convex hull and
parallel bodies of $\Om$ are both included in that of $\Om$.

Metric Geometry