Dynamic form help - javascript

Hi everyone.
I am trying to learn javascript but I am a little confused about something. I want to make a drop down list appear/disappear depending on which radio button is picked. here is a little example of what I mean.


<--all within a form-->
<input name="hotel" type="radio" value="Needed" checked>
YES, I Will Require a Hotel Room
<input name="hotel" type="radio" value="Not needed">
NO, I Do Not Require a Hotel Room

Arrival Date:
<select size="1" name="arrival">
<option value="June 15" selected>June 15</option>
<option value="June 16">June 16</option>
</select>                  

Departure Date:
<select size="1" name="departure">
<option value="June 16" selected>June 16</option>
<option value="June 17">June 17</option>
</select>

If possible I would like to make the two drop down lists “arrival” & “departure” appear only when the “Needed” radio button is checked.

Thanks for any help