Why rand() every time I refresh the page?

PHP Snippet 1:

<form>
    <?php echo $questionID; ?>
    <input type="text" name="answer" />
    <input type="hidden" name="question" value="<?php echo $questionID; ?>" />
</form>

PHP Snippet 2:

$questions  = array('dog legs','car legs','the best country in the world');
$answers    = array(4,4,'USA');
$questionID = rand(0, (count($questions)-1));

if (isset($_POST['answer'])) {
    $answer   = $_POST['answer'];
    $question = $_POST['question'];

    if ( $questions[ $question ] == $answer ) // you're golden
}

PHP Snippet 3:

<input type="hidden" name="questionId" value="<?php echo $questionId; ?>"></input>

PHP Snippet 4:

<?php
if ($_POST['questionID'])
    $questionID = $_POST['questionID'];
else
    $questionID = rand(0, (count($questions)-1));