\begin{frame}
  \frametitle{Vertical Line Test}
  
  When does a curve represent a function?
  \onslide<2->{
  \begin{block}{Vertical Line Test}
    A curve in the $xy$-plane represents a function of $x$ if and only if
    no vertical line intersects the curve more than once.
  \end{block}
  }
  
  \begin{center}
  \scalebox{.6}{
  \begin{tikzpicture}[default]
    \diagram{-.5}{5.5}{-.5}{5.5}{1}
    \diagramannotatez
    \draw[cblue,ultra thick,name path=curve] (0,1) to[in=180] (2,3) to[out=0,in=180] (4,2) to[out=0,in=-120] (5,3);
    \onslide<2->{
    \draw[cred,dashed,name path=line] (4,-.2) -- node[at start,below] {$a$} (4,5);
    \path [name intersections={of = curve and line}];
    \node[include=cred] at (intersection-1) {};
    \node at (2.5,-1.5) {corresponds to a function of $x$};
    }
  \end{tikzpicture}
  }\hspace{1cm}
  \scalebox{.6}{
  \begin{tikzpicture}[default]
    \diagram{-.5}{5.5}{-.5}{5.5}{1}
    \diagramannotatez
    \draw[cblue,ultra thick,name path=curve] (0,1) to[in=180] (2,3) to[out=0,in=180] (4,2) to[out=0,in=30,looseness=2] (3,4);
    \onslide<2->{
    \draw[cred,dashed,name path=line] (4,-.2) -- node[at start,below] {$a$} (4,5);
    \path [name intersections={of = curve and line}];
    \node[include=cred] at (intersection-1) {};
    \node[include=cred] at (intersection-2) {};
    \node at (2.5,-1.5) {does not correspond to a function of $x$};
    }
  \end{tikzpicture}
  }
  \end{center}
  \vspace{-1ex}
\end{frame}