\def\fifo#1{\ifx\ofif#1\ofif\fi
   \process#1\fifo}
\def\ofif#1\fifo{\fi}

\def\fifow#1 {\ifx\wofif#1\wofif\fi
  \processw{#1}\ \fifow}
\def\wofif#1\fifow{\fi}

\def\processw#1{\fifo#1\ofif}
\def\process#1{\boxit#1}
\def\boxit#1{\setbox0=\hbox{#1}%
  \hbox{\lower\dp0%
    \vbox{\offinterlineskip\hrule
      \hbox{\vrule\phantom#1\vrule}\hrule}%
  }%
}
\def\demobox#1{\leavevmode\fifow #1 \wofif \unskip{} }

\demobox{Tough exercise}