\begin{frame}
  \frametitle{Block Structures}

  \begin{exampleblock}{Allowed block structures}
    Blocks are allowed to be nested inside each other:
    \begin{center}
    \begin{tikzpicture}[thick]
      \draw (0,0) rectangle (1.5cm,2cm);
    \end{tikzpicture}
    \hspace{1cm}\pause
    \begin{tikzpicture}[thick]
      \draw (0,0) rectangle (1.5cm,2cm);
      \draw (.3cm,.3cm) rectangle (1.2cm,1.3cm);
    \end{tikzpicture}
    \hspace{1cm}\pause
    \begin{tikzpicture}[thick]
      \draw (0,0) rectangle (1.5cm,2cm);
      \draw (.3cm,.3cm) rectangle (1.2cm,.8cm);
      \draw (.3cm,1.1cm) rectangle (1.2cm,1.7cm);
      \draw (.5cm,1.3cm) rectangle (1cm,1.5cm);
    \end{tikzpicture}
    \end{center}
  \end{exampleblock}  
  \pause
  
  
  \begin{alertgoal}{}
    Blocks are not allowed to intersect:
    \begin{center}
    \begin{tikzpicture}[thick]
      \draw (0,0) rectangle (1.5cm,2cm);
      \begin{scope}[shift={(-.5cm,.5cm)}]
      \draw (0,0) rectangle (1.5cm,2cm);
      \end{scope}
    \end{tikzpicture}
    \hspace{1cm}
    \begin{tikzpicture}[thick]
      \draw (0,0) rectangle (.9cm,2cm);
      \begin{scope}[shift={(-.3cm,.5cm)}]
      \draw (0,0) rectangle (1.5cm,2cm);
      \end{scope}
    \end{tikzpicture}
    \end{center}
  \end{alertgoal}  
\end{frame}