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<body bgcolor="#cccc99" text="#000000" link="#000066" vlink="#000066" topmargin="5" bottomMargin="0" leftMargin="0"><table border="0" cellspacing="0" cellpadding="0" width="500"><tr><td align="center"><img src="/zimages70z/error/title.gif" height="54" width="284"><p>This feature of About.com requires frames.</p><p>click on the link below to proceed to <br /> <a href="http://www.pbs.org/teachersource/sci_tech.htm" target="_top">http://www.pbs.org/teachersource/sci_tech.htm</a>.</p></td></tr><tr><td align="center"><p>&nbsp<br><img src="/zimages70z/error/people1.gif" height="47" width="129"></td></tr></table></body>