  This paper studies a prototype of inverse initial boundary value problems
whose governing equation is the heat equation in three dimensions. An unknown
discontinuity embedded in a three-dimensional heat conductive body is
considered. A {\it single} set of the temperature and heat flux on the lateral
boundary for a fixed observation time is given as an observation datum. It is
shown that this datum yields the minimum length of broken paths that start at a
given point outside the body, go to a point on the boundary of the unknown
discontinuity and return to a point on the boundary of the body under some
conditions on the input heat flux, the unknown discontinuity and the body. This
is new information obtained by using enclosure method.
