WordPress & PHP: showing Hello World! only on Page 37?

I want to display a message on Page 37, only. I have tried various methods, including the following:

    $page_id=get_query_var('page_id');
    if ( $page_id == 37) {
        echo "Hello World!";
    }

But clearly, there is more to get_query_var(). It seems to hang WordPress.

What is the safest way to query the WP page for its ID?

Thanks