how to add an anti-spam challenge question to my contact form

<?php
//set flag to indicate whether mail has been sent
$mailSent=false;
if (array_key_exists('ewComments', $_POST)) {
  // mail processing script
// remove escape characters from POST array
if (get_magic_quotes_gpc()) {
  function stripslashes_deep($value) {
    $value = is_array($value) ? array_map('stripslashes_deep', $value) : stripslashes($value);
    return $value;
    }
  $_POST = array_map('stripslashes_deep', $_POST);
  }  
//validate the imput
$fname = trim($_POST['fristname']);
if (empty($firstname)){
$error['firstname']="Please enter your first name";
}
 
//validate the imput
$lname = trim($_POST['lastname']);
if (empty($lastname)){
$error['lastname']="Please enter your last name";
}
 
//validate the imput
$tel = trim($_POST['tel']);
if (empty($tel)){
$error['tel']="Please enter your telephone number";
}
 
$email = $_POST['email'];
// check for valid email address
$pattern = '/^[^@]+@[^\s\r\n\'";,@%]+$/';
if (!preg_match($pattern, trim($email))) {
  $error['email'] = 'Please enter a valid email address';
  }  
 
//check content of text area
$messageBody = trim($_POST['message']);
if (empty($messageBody)){
$error['message'] = 'Please enter your message';
}
 
  
//initialize variables
$to = 'geoffrey@geoffreyxlane.com';/*your email address*/
$subject = 'GXL Website Contact Form';
 
 
 
//build the message
$message = "First Name: $firstname\n\n";
$message .= "Last Name: $lastname\n\n";
$message .= "Email: $email\n\n";
$message .= "Telephone: $tel\n\n";
$message .= "Message: $messageBody";
 
// create additional headers
$additionalHeaders = 'From: GXL Contact Form';
if (!empty($email)) {
$additionalHeaders .= "\r\nReply-To: $email";
	  }
 
 
 
//create subject2
$subject2 = 'Thank you from Geoffrey X Lane';
 
//create message2
$message2 = '
Thank you for contacting Geoffrey X Lane. Geoffrey will be in touch with you within 24hours.
';
  
//create Header2
$additionalHeaders2 = 'From: geoffreyxlane.com';
 
 
//send the message if there are no errors
if(!isset($error)){
$mailSent = mail($to, $subject, $message, $additionalHeaders);
//check that mail was sent sucessfully
if(!$mailSent){
$error['notSent'] = 'Sorry there was a problem submitting your form. Please try again';
}
//send message to user
if(!isset($error)){
mail($email, $subject2, $message2, $additionalHeaders2);
 
}
 
}
}
?>
 
<!--
function MM_validateForm() { //v4.0
  if (document.getElementById){
    var i,p,q,nm,test,num,min,max,errors='',args=MM_validateForm.arguments;
    for (i=0; i<(args.length-2); i+=3) { test=args[i+2]; val=document.getElementById(args[i]);
      if (val) { nm=val.name; if ((val=val.value)!="") {
        if (test.indexOf('isEmail')!=-1) { p=val.indexOf('@');
          if (p<1 || p==(val.length-1)) errors+='- '+nm+' must contain an e-mail address.\n';
        } else if (test!='R') { num = parseFloat(val);
          if (isNaN(val)) errors+='- '+nm+' must contain a number.\n';
          if (test.indexOf('inRange') != -1) { p=test.indexOf(':');
            min=test.substring(8,p); max=test.substring(p+1);
            if (num<min || max<num) errors+='- '+nm+' must contain a number between '+min+' and '+max+'.\n';
      } } } else if (test.charAt(0) == 'R') errors += '- '+nm+' is required.\n'; }
    } if (errors) alert('The following error(s) occurred:\n'+errors);
    document.MM_returnValue = (errors == '');
} }
 
/*form*/
 
<?php if(isset($error['notSent'])){ ?>
      <h5 class="warning">SERVER ERROR</h5>
      <p class="warning"><?php echo $error['notSent'];?></p>
      <?php } elseif ($mailSent) { ?>
      <H5 class="warning">THANK YOU!</H5>
      <p> Thank you for your inquiry. Geoffrey will be in touch with you shortly.</p>
      <?php } else { ?>
      <p>&nbsp;</p>
      <?php } ?>
      <form action="<?php $_SERVER['PHP_SELF'];?>" method="post" name="contactForm" id="contactForm" onSubmit="MM_validateForm('firstname','','R','lastname','','R','tel','','R','email','','RisEmail');return document.MM_returnValue">
        <p>
          <label for="fname"><span>First Name:</span></label>
          <?php if (isset($error['firstname'])){ ?>
          <span class="warning"><?php echo $error['firstname']; ?></span>
          <?php } ?>
          <input type="text" name="firstname" id="firstname"
      <?php if(isset($error)) {echo "value='$firstname'";}?>/>
        </p>
        <p>
          <label for="lastname"><span>Last Name:</span></label>
          <?php if (isset($error['lastname'])){ ?>
          <span class="warning"><?php echo $error['lastname']; ?></span>
          <?php } ?>
          <input type="text" name="lastname" id="lastname"
      <?php if(isset($error)) {echo "value='$lastname'";}?>/>
        </p>
        <p>
          <label for="tel"><span>Telephone:</span></label>
          <?php if (isset($error['tel'])){ ?>
          <span class="warning"><?php echo $error['tel']; ?></span>
          <?php } ?>
          <input type="text" name="tel" id="tel"
        <?php if(isset($error)) {echo "value='$tel'";}?>
      />
        </p>
        <p>
          <label for="email"><span>Email:</span></label>
          <?php if (isset($error['email'])){ ?>
          <span class="warning"><?php echo $error['email']; ?></span>
          <?php } ?>
          <input type="text" name="email" id="email"
       <?php if(isset($error)) {echo "value='$email'";}?>/>
        </p>
        <p>
          <label for="message">Comments/Questions:</label>
          <?php if (isset($error['message'])){ ?>
          <span class="warning"><?php echo $error['message']; ?></span>
          <?php } ?>
          <textarea name="message" id="message" cols="30" rows="5"><?php if(isset($error)){echo $messageBody;}?>
</textarea>
        </p>
        <p>
          <label for="ewComments"></label>
          <input type="submit" name="ewComments" id="ewComments" value="Send" />
        </p>
      </form>