Have you ever sealed an envelope only to realize that you forgot something, such as signing a check or including a rebate form? Normally you would have to open it and throw the used envelope away, right? Well here is a handy trick that you can use to save that envelope.
Put it into the freezer for a few hours. Take it out and then run a knife under the flap. It should pop open very easily. The glue will still be good, so you can seal it right back up again. This will also work with new envelopes that might have become accidentally sealed by humidity.