While reading someone else's mail is generally not advised, there may be instances where you want to open a sealed envelope without evidence that the seal was tampered with. Whatever your reason for doing so, there are ways to secretly open a sealed envelope.
Using Steam To Dissolve The Glue
The most time-honored tradition of secretly opening envelopes involves holding them over a kettle of boiling water. You can stick a spoon in the kettle's spout to keep steam coming out in a straight line. The steam will dissolve the glue holding the letter together and allow you to open it without evidence of tearing.
Once the kettle is steaming, find an oven glove to keep your hand from getting burned, as you may have to hold the envelope directly in the path of the steam for up to half a minute before the glue dissolves. Once you have your glove, establish a firm grip on the letter you wish to open and dangle it over the steam for fifteen seconds. If the paper or envelope seems to be getting drenched, take it out immediately and let it dry on a flat surface.
Opening The Letter
Wait about 15 seconds for the letter to cool down and dry off, then take a thin knife or letter opener and gently pass it under the seal. The flap should open without any tearing or ripping. If there is any resistance whatsoever, hold the letter under the steam for another 10-15 seconds and try to open the letter again.
Once open, carefully take the letter out of the envelope. You may want to wear rubber or latex gloves to avoid getting smudges on it. Take note of how the letter was folded so you can place it back exactly as you took it out. Do not introduce any new folds or crumple marks to the letter. Read the letter or take a picture of it for your records, then carefully fold it back and place it in the envelope.
Re-Sealing The Envelope
The whole point of secretly opening a sealed envelope is to close it and make it seem as though you didn't read the letter. Apply a very light amount of glue with a thin glue stick or strip of rubber cement to the edges of the letter, then carefully close the envelope over the adhesive. It is crucial that you not get smudges of glue on the back or sides of the letter, as this could indicate to others that the envelope was already opened.