At some point, just about everyone has written a letter and sealed it up only to realise that they left out something important. A thought left unsaid, a signature forgotten or a picture that was meant to be included are just a few of the reasons why you might find yourself in need of opening a sealed envelope. While replacing the envelope isn't overly expensive on its own, if you consider the cost of replacing a stamp and the hassle of readdressing a new one, you might want to consider alternate methods of getting it open with breaking or tearing the seal.
The Boiled Water Method: Boil water to create steam. Fill a small pot with water from the tap or fill a tea kettle. Place the pot or kettle on the stove until the pot comes to a full boil or the tea kettle begins to whistle. Hold your envelope seam side down over the steam with kitchen tongs or a mitten pot holder to prevent the heat from burning your hands. After 10 seconds, gently run your finger or the blade of a knife under the flap of the envelope to gently part the loosened seal. Place over the steam for another 10 seconds as needed if the seal is not loose enough to open without tearing.
The Steam Iron Method: Add water to your steam iron. Plug in and heat up your iron the same way you would if you were steaming the wrinkles from clothing. When ready, run the iron just above the seam of the envelope for several seconds and allow the steam to loosen the seal. Be careful not to allow the heat from the iron to singe or discolour the envelope. Gently pry the envelope open using your fingers, knife or the sharp edge of a pair of scissors.
The Freezer Method: Freeze the envelope open. Place the sealed envelope in your freezer. Allow the envelope to sit undisturbed for 1 to 2 hours, the side with the seal facing up. Remove the letter from the freezer and use a kitchen knife to separate the seal if it does not automatically open on its own. If it does not easily open, place the envelope back in the freezer for another half an hour before attempting to open it again.
Tampering with another person's mail is against the law and can result in jail time. You should use these techniques only when you are reopening your own mail. The steam will make the envelope moist and susceptible to tearing. Use extreme caution to avoid ripping the letter and the envelope.