Conditional Tags

I found this on the Codex page for is_category tag:
<?php $post = $wp_query->post; if ( in_category('1') ) { ?>
that particular header
<?php } ?>
I tried that in my Post template, single.php, (inside the loop, despite the header in Codex) and it worked.

What site are you working on? The one linked from your name? Can you point to an example where it’s supposed to be working? Where did you put that bit of code?

Is the file where include() expects it to be? Does it have adequate permissions?

Could you test if you can get *anything* (normal text) printed on the page with the code snippet?