HTTP headers and include()

Hey everyone, I've been hard at work on a new web application, and discovered something that I would never have seen coming. I was noticing that when I called session_start() after a few lines of includes, I was getting complaints because the HTTP headers had already been sent out. Then, after putting session_start() above the include lines, suddenly everything was working fine. The files that were included were nothing more than functions; there was no code executing that I could tell up to the point of the call to session_start(). I was just wondering if anybody on the list knows why HTTP headers were being sent out by my includes. I'm sure there's a good reason. I'm just very curious :) Thanks very much in advance. James