The easiest way to open an envelope is to steam it over a boiling pot of water. When the envelope is opened in this way, it can be resealed as if it were never opened in the first place.
By boiling a pot of water and holding the envelope over the steam from the water, the seal of an envelope will be broken. The glue that is used in envelopes is sensitive to both heat and moisture and steam will provide both. It is important that the envelope is not held too closely to the boiling water because it could easily become damaged. Once the envelope has been opened, it can easily be resealed by applying a light amount of glue from a glue stick.
When opening and resealing an envelope, the most important thing is to not cause any damage to the envelope. Be sure that the envelope is not wrinkled or bent in the process. Be extremely careful when opening the seal after steaming it because some parts of the glue may be undone while other parts may still be sealed. Make sure that the envelope is completely open before attempting to look inside of it.
There are many reasons that an envelope would need to be opened again after it has been sealed once. The most common reason is that something was forgotten inside the envelope and the stamp has already been placed on it. It is important to note that it is illegal to open another person's mail and an envelope should never be opened and resealed for this reason.