Solution:
RSS feeds are XML. When you want to include character data in a feed without making sure that data is valid XML, you can enclose it in a CDATA section.
If get_daily_fun_fact() returns HTML output, you can include that in an RSS element with this formatting:
<title>
<![CDATA[<?php get_daily_Fun_Fact(); ?>]]>
</title>
A CDATA section begins with <![CDATA[ and ends with ]]> Everything between those is assumed to be character data instead of XML markup.