I was getting ready to mail something to a friend. I sealed and addressed the envelope, added a stamp and was walking out to the mailbox when I realized that part of what I needed to send was still sitting on my desk! I didn't want to walk downstairs to get another envelope and waste a stamp. Instead, I steamed the envelope open.
I put a pot of water on the stove. When it boiled, I held the envelope over the steam (flap-side down) for about 15 seconds, then tried to lift the flap. It didn't work, so I steamed it for 15 seconds more. This time the glue was soft, and half of the flap came loose. However, when I got to the middle of the flap it started tearing. I put the envelope back over the steam, checking the glue every so often. It took longer than I expected, but after a minute or so the glue finally released and I was able to open the flap completely.
I added the missing papers to the envelope and sealed it up again with some glue. The flap of the envelope was all wrinkled when I was done. If I ever want to have a career as a spy, I'll have to work on my steaming skills.