[Tex/LaTex] Sums in math mode: how to display index under the sigma sign

Question

I have a fraction with sums above and under the line. How can I convince LaTeX to write the indices of the sums under the sigma instead of next to it?

\begin{displaymath}
  \frac{\sum_{s \in S} s^2}{\sum_{p \in P} p^2}
\end{displaymath}

Answer 1

\begin{displaymath}
  \frac{\sum\limits_{s \in S} s^2}{\sum\limits_{p \in P} p^2}
\end{displaymath}