Beamer - Unhide Content at click

This is very cheap, but at the same time very interactive.

\documentclass{beamer}
\usepackage{pdfcomment}

\begin{document}
    \begin{frame}
        \begin{block}{Questions}
            \begin{itemize}
                \item A 
                    \pdfsquarecomment
                        [height=.8cm,width=5cm,voffset=-.2cm,hoffset=-.4cm]
                        {use mouse to move this}
                \item B
                \item C
                \item D
                \item E
                \item ...
            \end{itemize}
        \end{block}
    \end{frame}
\end{document}

This idea is to build an annotation that covers the contents. When the audience mentions it, use your mouse to move the annotation away. (Or simply press delete or backspace to delete it)

Advantage

Since only annotation involves, this is supported by more PDF readers. (In contrast, PDF readers from the unix world does not like javascript and a bunch of useful PDF features. This includes Preview on macOS.)

Bonus

This is called Tangram.

Have fun!

\documentclass{beamer}
\usepackage{tikz,pdfcomment}

\begin{document}
    \begin{frame}
        \pdflinecomment[type=polygon,line={300 100 350 150 300 200}]{}
        \pdflinecomment[type=polygon,line={150 60 250 60 200 10}]{}
        \pdflinecomment[type=polygon,line={10 50 60 50 60 100}]{}
        \pdflinecomment[type=polygon,line={10 210 35 235 60 210 35 185}]{}
        \pdflinecomment[type=polygon,line={150 200 175 225 175 275 150 250}]{}
        \pdflinecomment[type=polygon,line={300 20 350 20 325 45}]{}
        \pdflinecomment[type=polygon,line={260 190 285 215 285 165}]{}
        \tikz[remember picture,overlay]{
            \draw(current page.center)+(-50bp,-50bp)rectangle+(50bp,50bp);
        }
    \end{frame}
\end{document}

Using the ocgx package mentioned in the link posted in Andrew's comment I managed to solve the problem:

\documentclass[ngerman]{beamer}
\usepackage{ocgx}
\begin{document}
\begin{frame}
\begin{block}{Questions}
\begin{itemize}
\item \begin{ocg}{Bul01}{ocg01}{0}\actionsocg{ocg01}{}{}{A}\end{ocg}
\item \begin{ocg}{Bul02}{ocg02}{0}\actionsocg{ocg02}{}{}{B}\end{ocg}
\item \begin{ocg}{Bul03}{ocg03}{0}\actionsocg{ocg03}{}{}{C}\end{ocg}
\item \begin{ocg}{Bul04}{ocg04}{0}\actionsocg{ocg04}{}{}{D}\end{ocg}
\item \begin{ocg}{Bul05}{ocg05}{0}\actionsocg{ocg05}{}{}{E}\end{ocg}
\item \begin{ocg}{Bul06}{ocg06}{0}\actionsocg{ocg06}{}{}{...}\end{ocg}
\end{itemize}
\end{block}
\end{frame}
\end{document}