Keep position of graphic in beamer

Question

Possible Duplicate:
Avoiding jumping frames in beamer

how can I set a figure or a graphic to keep it's position across multiple slides in a presentation?

I'm using the \only command to show only an excerpt of an explanation underneath a figure/graphic. Since these excerpts don't have the same height, the figure/graphic's position changes. But I want the figure/graphic to keep its position and I don't want to use \uncover.

What do I need to do?

Here is an example where a rectangle represents my figure/graphic which doesn't hold its position.

\documentclass[10pt]{beamer}

\usepackage[utf8]{inputenc}
\usepackage{tikz}

\begin{document}
\begin{frame}\frametitle{Test}
\begin{center}
  \tikz\draw (0,0) rectangle (2,2);
\end{center}

Explanation:
\only<1-1>{
  \begin{itemize}
    \item[step 1]
    \item step, step, step
  \end{itemize}
}
\only<2-2>{
  \begin{itemize}
    \item[step 2]
    \item step two
    \item some more
    \item and even more
  \end{itemize}
}
\end{frame}
\end{document}