Latex doesn't create new page when using section? [duplicate]

I'm trying to create my document with lots of section. However, after the document is compiled by pdfLaTeX, the document showed like this:

\documentclass{article}
\begin{document}
\section{aaa}  % * a lots of time
\end{document}


How to make sure the sections doesn't goes out of bond and create new page automatically?