<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" onclick="displayIceCreamOptions()"/><label for="ic_yes">Yes</label><br/>
<input type="radio" name="want" id="ic_no" value="no" onclick="displayIceCreamOptions()"/><label for="ic_no">No</label></div><fieldset id="icecream_options" style="display: none;"><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 {
    display:none; 
    background-color: #eee;
    margin-left: 2em;
}

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

function displayIceCreamOptions() {
    var divico = document.getElementById('icecream_options');
    var inputYes = document.getElementById('ic_yes');
    if (inputYes.checked == true) {  
      divico.style.display = 'block'; 
    } else { 
      divico.style.display = 'none'; 
    }
}