The TeXbook exercise 11.5

exercise11.5.tex

%#!latex
\documentclass{article}

\makeatletter
\def\demobox{\bgroup\catcode`\ =12\relax\xx@demobox}
\def\xx@demobox#1{%
  \fboxsep=0pt\fboxrule=.4pt
  \fbox{%
    \fboxrule=3\fboxrule
    \xx@demob@x#1\relax}%
  \egroup}
\def\xx@demob@x#1{%
  \ifx#1\relax\else
    \if\space#1\relax\space\else\fbox{\phantom{#1}}\fi
    \xx@hop\xx@demob@x
  \fi}
\@gobbletwo\if\if\def\xx@hop#1\fi{\fi #1}
\makeatother

\begin{document}
\demobox{Tough exercise.}
\end{document}