\begin{frame}
  \frametitle{Modal Logic}
  \bigskip

  \begin{exampleblock}{}
    Which of the following formulas are valid?
    \begin{itemize}
    \smallskip
    \item $\all p \ifo  \neg\some\neg p$
    \smallskip
    \item $\some p \wedge \some q \to  \some (p\wedge q)$
    \smallskip
    \item $\all p \to  \some p$
    \smallskip
    \item $\all p \to  p$
    \smallskip
    \item $\all (p\to q)  \to ( \all p \to \all q)$
    \smallskip
    \item $\some \some p \to \some p$
    \smallskip
    \item $\all\,\neg \perp$
    \smallskip
    \end{itemize}
  \end{exampleblock}
 \pause
 \bigskip

 That depends on the interpretation of the modal operators!
\end{frame}