Display Form Object When Certain Option Chosen From Select Tag

I am building a form with ColdFusion, HTML, and will be using some Javascript, and I wnt the javascript to tell one form object to display itself when a certain option is chosen from a <select> tag.

<cfquery name="getCategories" datasource="#mydatasource#" username="#myusername#" password="#mypassword#">
SELECT * FROM categories WHERE type = 'cat' ORDER BY id
</cfquery>
<cfform action="engine.cfm" method="post" format="html">
<table cellpadding="0" cellspacing="0">
<tr><th colspan="2">New Category</th>
</tr><tr>
<td>Category Type:</td>
<td><select name="type">
<option value=" "></option>
<option value="cat">Main Category</option>
<option value="sub">Sub Category</option>
</select></td></tr>
<tr><td>Name:</td>
<td><cfinput type="text" name="title" size="20" maxlength="100" required="yes" message="A name for the category is required.">
</td></tr>
<tr><td>Parent Category:</td><td>
<cfselect enabled="Yes" name="parentcat" multiple="no" query="getCategories" value="id" display="title" queryPosition="below">
<option value=" "></option></cfselect>
</td></tr><tr>
<td colspan="2"><cfinput type="submit" name="submit" value="Submit" validate="submitonce" validateat="onserver,onsubmit"></td>
</tr></table></cfform>

I want the <cfselect> tag to display on if Sub Category is selected on the first <select> tag. I expect that it will use the onchange attribute but other than that I have no idea.