Keep in mind that the steam method can get a little messy and potentially warp the paper, so if it's imperative that the envelope looks fresh and new, you might want to just use a new envelope instead. [substeps] If the steam is coming out in a strong, steady stream, stick a spoon in the spout to diffuse it.
[header] How to secretly open a sealed envelope [title] Place a tea kettle on the stove. [step] Boil water in a tea kettle until steam begins to shoot through the spout. The steam is what you'll use to loosen the glue in the envelope you want to open.