\begin{frame}
  \frametitle{Propositional Logic: Introduction}
  
  \begin{goal}{}
    Propositions are ``declarative sentences''.
  \end{goal}
  \bigskip
  
  \begin{exampleblock}{Examples}
    \begin{mgather}
      \text{It rains.}\\
      8 > 5\;.\\
      6 < 2\;.\\
      \text{$X$ is positive.}\\
      \text{The earth is flat.}
    \end{mgather}
  \end{exampleblock}
\end{frame}