echo "one foo=bar" gives an unexpected result. Why ?

Hello all.

I was thinking about how some characters are not allowed in filenames, and
tried to see if "=" is one of them.
So, I entered, on a commandline, the following: "one foo=bar".

To my surprise the contents of the file became "one =bar"

Can anybody tell me why that "one =bar" result ? (googeling is a bit hard on
it, with the lack of keywords).

Regards,
Rudy Wieser