Javascript: explain this one line of code to me please
<p>I have this simple array:</p>

<pre><code>var RedirUrl = new Array(4);
RedirUrl[0] = 'http://mafi.se/mf_redir/new_install_'+this_version+'.html';
RedirUrl[1] = 'http://ifurls.com/mf_redir/new_install_'+this_version+'.html';
RedirUrl[2] = 'http://ez.se/xml-update/mf_redir/new_install_'+this_version+'.html';
RedirUrl[3] = 'http://ilovre.net/mf_redir/new_install_'+this_version+'.html';
RedirUrl[4] = 'http://rihel.com/mf_redir/new_install_'+this_version+'.html';
</code></pre>

<p>and then</p>

<pre><code>RedirUrl.sort(function() {return 0.5 - Math.random()}) 
</code></pre>

<p>The last bit is what is confusing me.
I understand the "sort" and I understand the <code>Math.random</code> but the <code>return 0.5</code> confuses me... what exactly is that?</p>

<p>(Needless to say I downloaded it off the net as it does what i want it to do... but i just dont understand it.)</p>
