Not to blow anyone's trumpet, but this may actually be less secure, as there is always the possibility that the postman/postwoman/postperson will open your letter and read them, then replace them in a different envelope. They could also shine bright light through the envelope to get a better view of the writing, or possibly use some form of scanning such as MRI?

Please read some of these methods that people have suggested on the internet from 2008, which I have found through Google.

i think the actual "classic" way probably doesn't involve a microwave... try ironing the letter with a lot of steam. but of course opening a sealed envelope is very easy. reclosing it once you've opened it is the real trick. consider the attentiveness of your audience. will the person whose mail you're reading notice if you open really carefully and then re-seal with a glue stick? probably no...

If it’s not a heavy or security-print envelope, throw it on a backlit scanner (any flatbed scanner with a nice, bright transparency lamp should work) and get a 24-bit color scan. Open the scan in Photoshop and manipulate the gamma correction or color correction curves to maximize the contrast between the envelope and the text inside. Rotate as necessary. You can usually get something and you don’t even have to break the seal. This, of course, is unlawful and unethical, and should never actually be done.

Just open it up, read it, then destroy it. The recipient doesn't need to know that they had a piece of mail.

Upside down can of computer duster air works wonders in making the envelope transparent. Then it evaporates, leaving no traces.

Edit: Dentists can also discreetly read your mail.

:D /me offers a hug The world is a dark place, but don't let discourage you :)