Why do Objective-C files use the .m extension?

Question

Since I started learning Objective-C and Cocoa, I\'ve been wondering why they have chosen the extension .m for the implementation files - was it supposed to mean something,

Answer 1

Today most people would refer to them as "method files", but

"The .m extension originally stood for "messages" when Objective-C was first introduced, referring to a central feature of Objective-C [...]"

(from the book "Learn Objective-C on the Mac" by Mark Dalrymple and Scott Knaster, page 9)

EDIT: To satisfy an itch I emailed Brad Cox, the inventor of Objective-C, about the question and he answered with this single line:

"Because .o and .c were taken. Simple as that."

Here's the email as visual proof: