Devilishly cynical, it's akin to hacking snail mail. Envelope X-Ray Spray does exactly what it says on the can, turning paper temporarily translucent. 30 seconds after application, it vanishes, leaving no trace on the envelope which will have returned to its original state. It's eco-friendly too, though we're guessing if you're the kind of person who goes snooping through other people's post, you're probably not too concerned about the welfare of a few trees now, are you?