stop mail including headers on forward

I have the latest version of Mountain Lion. While this was not happening after the upgrade, at somepoint along the way something changed--by me or by Apple update. I want to change it back.

Details: When I forward mail, Mail includes the full email header at the top of the email content. Not just From and To and what was there in the past, but *all* the headers including Content-Type,Content-Transfer, Message-ID,X-Mailer,Mime-Version, all the Delivered-To, Recived, ... everything! It it put at the top of the email. Using classic or the "new" view makes no difference. Happens whether or not "details" are shown when viewing the mail in the pane. Does not happen when "reply" to email. Just with "forward".

How to turn this feature off? I do not want to include headers like this. I've checked all the Preferences and Menus and cannot find anything which controls this behaviour. If there, it's hiding in plain sight for me. I don't recall turning it on. This happens whether the "details" are "on" or "off" when viewing the original mail.

Help.