% This picture represents the fact that reflections are rigid motions
% (C) Thiago Brevidelli
\begin{tikzpicture}
  % The first square
  \begin{scope}[shift={(1.5, -1)}]
    \draw (0, 0) -- ++( 2, 0)
                 -- ++( 0, 2)
                 -- ++(-2, 0)
                 -- cycle;

    \draw[thick] (0, 2) -- (1, 0);
    \draw[thick] (1, 2) -- (2, 0);
    \draw (0, 0) -- (2, 2);
  \end{scope}

  \draw (1, 0) node{$\leadsto$};

  % The seccond square
  \begin{scope}[shift={(-1.5, -1)}]
    \draw (0, 0) -- ++( 2, 0)
                 -- ++( 0, 2)
                 -- ++(-2, 0)
                 -- cycle;

    \draw[thick] (0, 0) -- (1, 2);
    \draw[thick] (1, 0) -- (2, 2);
    \draw (0, 2) -- (2, 0);
  \end{scope}
\end{tikzpicture}