To recycle bits (or recycle electrons) is to take an old piece of email and use it to answer a similar (often identical) question or discussion on a mailing list. This is usually done by simply replying to the thread with the two-word message "Recycling bits" (or "Recycling electrons") and attaching the original email message.

An important aspect of the use of this term is that the attached email message definitively answers the question or resolves the discussion. Usually, the attached email message comes from the very same mailing list that is hosting the current discussion. For example, consider this question:

From: X
To: Q Discussion
Subject: How do I do X?
Date: February 8, 2011

Blah blah blah blah? Blah blah. Thanks.

An example reply which earns maximum style points would go something like this:

From: Y
To: Q Discussion
Subject: RE: How do I do X?
Date: February 8, 2011

Recycling electrons.

<Attached message>

From: S
To: Q Discussion
Subject: RE: How do I do X?
Date: October 31, 2010

You first need to frob the blurble and then blah blah blah blah.

----Original message----

From: R
To: Q Discussion
Subject: How do I do X?
Date: October 30, 2010

Blah blah blah blah? Blah blah. Thanks.

Since the question asked on February 8 is identical to the question asked on October 30, the answer from S can be re-used verbatim. (This of course assumes that nothing has happened in between to invalidate S's original answer.)

See also: "I refer the honourable gentleman to the answer given some moments ago."