Nobody likes a snoop. But if you’ve somehow managed to come up with a reason to justify why you need to go snooping into someone else’s mail, then this little can of Envelope X-ray Spray might help you out. I think the name of the product says it all, actually.
Some people advise holding a sealed envelope over steam to open the adhesive, so you can check the contents of the envelope without damaging it in the process. But if boiling water is a task you find too tedious and risky, then this spray is your next best alternative. Even though the makers of the X-ray Spray insist that it won’t damage or discolor the envelope or its contents, you can never be too sure – so use it at your own peril.
Basically, spraying this “magic fluid” (likely some kind of alcohol and propellant) over the envelope will make that area less opaque. It kind of looks like you got it wet with water, only it’s not water.