  Now a final and maybe simplest formulation of the enclosure method applied to
inverse obstacle problems governed by partial differential equations in a {\it
spacial domain with an outer boundary} over a finite time interval is fixed.
The method employs only a single pair of a quite natural Neumann data
prescribed on the outer boundary and the corresponding Dirichlet data on the
same boundary of the solution of the governing equation over a finite time
interval, that is a single point on the graph of the so-called {\it response
operator}. It is shown that the methods enables us to extract the distance of a
given point outside the domain to an embedded unknown obstacle, that is the
maximum sphere centered at the point whose exterior encloses the unknown
obstacle. To make the explanation of the idea clear only an inverse obstacle
problem governed by the wave equation is considered.
