When Jayne receives the message from Cynthia, she'll see something like this:

Jayne can then put her cursor in the body of the message she received and hit Ctrl+D. She should see a message similar to this one, stating that the PGP signature has been verified. She knows that the message really came from Cynthia, and hasn't been altered since Cynthia PGP-signed it.

If, however, the text had been altered in any way - even with one space added or removed, or a capital letter changed to a lowercase letter - the signature would come up as invalid. In this case, Cynthia had simply forgotten something she meant to say, and added it after the signing the message - but she forgot to erase the now-invalid signature and re-sign it. So the signature is invalid.

Next: Encrypting Messages with PGP