<form method="get" name="ice_cream" id="ice_cream" action="https://cscie12.dce.harvard.edu/echo">
   <div>Would you like ice cream?
     <br/>

     <label>
       <input type="radio" name="want" id="ic_yes" value="yes"/>
Yes     </label>
     <label>
       <input type="radio" name="want" id="ic_no" value="no"/>
No     </label>
   </div>
   <fieldset id="icecream_options">
     <legend>Ice Cream Options     </legend>
     <p>How would you like it?
     </p>
     <label>
       <input type="radio" id="container_cup" name="container" value="cup"/>
Cup     </label>
     <br/>

     <label>
       <input type="radio" id="container_cone" name="container" value="cone"/>
Cone     </label>
     <br/>

     <p>Pick your toppings:
     </p>
     <label>
       <input type="checkbox" name="toppings" id="toppings_wc" value="whipcream"/>
Whipped cream     </label>
     <br/>

     <label>
       <input type="checkbox" name="toppings" id="toppings_j" value="jimmies"/>
Jimmies     </label>
     <br/>

     <label>
       <input type="checkbox" name="toppings" id="toppings_nuts" value="nuts"/>
Nuts     </label>
     <br/>

     <label>
       <input type="checkbox" name="toppings" id="toppings_cherry" value="cherry"/>
Cherry     </label>   </fieldset>
   <p>
     <input type="submit"/>

   </p>
 </form>   

In style element (<style>) within head element:

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

In script element within head element (<script>):

$(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');
    }
}