<form method="get" name="ice_cream" id="ice_cream" action="http://cscie12.dce.harvard.edu/echo">
<div>Would you like ice cream?<br/>
<input type="radio" name="want" id="ic_yes" value="yes"/><label for="ic_yes">Yes</label><br/>
<input type="radio" name="want" id="ic_no" value="no"/><label for="ic_no">No</label></div><fieldset id="icecream_options"><legend>Ice Cream Options</legend><p>How would you like it?</p>

<input type="radio" id="container_cup" name="container" value="cup"/><label for="container_cup">Cup</label><br/>
<input type="radio" id="container_cone" name="container" value="cone"/><label for="container_cone">Cone</label><br/><p>Pick your toppings:</p>

<input type="checkbox" name="toppings" id="toppings_wc" value="whipcream"/><label for="toppings_wc">Whipped cream</label><br/>
<input type="checkbox" name="toppings" id="toppings_j" value="jimmies"/><label for="toppings_j">Jimmies</label><br/>
<input type="checkbox" name="toppings" id="toppings_nuts" value="nuts"/><label for="toppings_nuts">Nuts</label><br/>
<input type="checkbox" name="toppings" id="toppings_cherry" value="cherry"/><label for="toppings_cherry">Cherry</label></fieldset><p>
<input type="submit"/></p>

</form>
   

In style element (<style type="text/css">) within head element:

#icecream_options { 
    background-color: #eee;
    margin-left: 2em;
}

In script element within head element (<script type="text/javascript">):

$(document).ready(function(){
    $("#icecream_options").hide();
    $("input[name='want']").click(function(){
      displayIceCreamOptions();
    });
});
function displayIceCreamOptions() {
    if ($("input[name='want']:checked").val() == 'yes') {
      $('#icecream_options').fadeIn('slow');
    } else {
      $('#icecream_options').fadeOut('slow');
    }
}