function riddler_captcha($op, $captcha_type='') {
  switch ($op) {
    case 'list' :
      return array('Riddler');
      break;
    case 'generate' :
      if ($captcha_type == 'Riddler') {
      $result = array();
      $riddles = riddler_get_riddles();
      $key = array_rand($riddles);
      $result['form']['captcha_response'] = array(
        '#type' => 'textfield',
        '#title' => $riddles[$key]['question'],
        '#description' => t('Fill in the blank.'),
        '#size' => 50,
        '#maxlength' => 50,
        '#required' => TRUE,
        '#weight' => variable_get('riddler_weight', 0),
      );
      $result['solution'] = (string)(drupal_strtolower($riddles[$key]['answer']));
      $result['captcha_validate'] = 'riddler_captcha_validate';
      return $result;
      }
    break;
  }
}