[lug] gpg question

[Scott A. Herod]

Hello,

I'm trying to figure out gnupg finally and was curious about signing
files.  Is there a way to generate an external signature file which
verifies some file?  In particular, I create a file foo.txt which I 
want to sign.  I wish to send the file foo.txt and the signature file,
presumably foo.txt.asc, so that the recipient can verify that yes, 
I sent them the files and furthermore the foo.txt they received is
the one I sent.

"gpg -sa foo.txt" appears to generate the signature file but
"gpg --verify foo.txt.asc foo.txt" errors and
"gpg --verify foo.txt.asc" says ok regardless of whether I edit
foo.txt after creating the .asc file.

What is the correct way to do this?

Thanks

Scott