Animation on beamer

Two possibilities, depending on the intended overlay format:

\documentclass{beamer}

\begin{document}

\begin{frame}
\only<1>{first}\only<2>{second}

\onslide<3>{third} \onslide<4>{fourth}
\end{frame}

\end{document}

You can also use \pause:

\begin{frame}
first\pause second
\end{frame}

You could also take a look at the beamer manual for \alt and \temporal.

Here's an example of several overaly specifications:

\documentclass{beamer}

\begin{document}

\begin{frame}
\begin{itemize}
\item<1-> \only<1>{first}\only<2->{second}
\item<3-4> \onslide<3->{third}\onslide<4>{ fourth}
\end{itemize}
\end{frame}

\end{document}

Try this:

\begin{frame}
  \only<1-2>{show_me_first}
  \only<2>{show_me_second}
\end{frame}

Best regards, Charlie