[squid-users] 'file' won't cache

I've got a web site that dynamically generates results to a request.
This request will be highly utilized, but it needs to remain fairly
fresh. So, I want squid to cache it for only about 19 seconds.
However, squid won't cache it.

Here's what I'm seeing when I make the request:

----
   Date: Tue, 08 Aug 2006 17:58:10 GMT
   Content-Length: 3055
   Content-Type: text/plain
   Cache-Control: max-age=0, s-maxage=19, must-revalidate, proxy-revalidate
   X-Cache: MISS from xxx.org
   X-Cache-Lookup: MISS from xxx.org:80
   Connection: keep-alive
-----
If I perform a request one after another, the request still isn't 
cached.  However, if I change 's-maxage' to 60 seconds, it does get 
cached.  The requests I'm making to see if caching takes place are 
within seconds of each other.
Any ideas?