set_transient and get_transient – does not work for me

Hi!

I want to try out the set and get_transient functions. As an example I used my tag_cloud which is generated randomly on every page request. I set the timeout to 10 seconds but it does not seem to work.

What am I doing wrong?

// Get any existing copy of our transient data
// delete_transient('special_query_results');
$special_query_results = get_transient('special_query_results');

// If it wasn't there regenerate the data and save the transient
if (!$special_query_results) {
    echo "<!-- caching -->";
$special_query_results = wp_tag_cloud(array(
    'smallest'  => 8,
    'largest'   => 16,
    'number'    => 30,
    'order'     => 'RAND',
    'echo'      => false ));
     set_transient('special_query_results', $special_query_results, 10);
}
echo $special_query_results;