Checkmate

I have a couple of friends who are serious practical jokers, unfortunately, there's nothing practical about their jokes which usually cause more harm than smiles. I suspected that they were up to something, so I devised a plan to beat them to the punch. I was staying at my brother-in-law's home in Salt Lake, and invited them to pay me a visit. When they arrived, I had them sit down while I went outside for a moment. When I returned, I was wearing latex gloves, and holding two white business size envelopes. I handed the envelopes to each of them, and they immediately opened them, and removed the contents, a blank white piece of paper in each. They looked closely to see if there was something they were missing. I asked, "How are you at reading between the lines?" With confused expressions, they each held their blank white paper up to the light hoping to reveal the mystery. After sufficient handling, I gathered their papers, returned them to their respective envelopes, took them back outside, and secured them in the trunk of my car. When I returned, I removed the latex gloves, and said, "Checkmate." They looked at each other wondering, had they been had, and what was I was up to? "I want to thank you guys for putting your fingerprints all over those envelopes and papers." I said. Suddenly they looked concerned... they knew they'd been had. "And if you guys ever play one of your little jokes on me, you never know who might receive a threat letter addressed from you with your grubby little fingerprints are all over it." I said. They looked at each other rather sheepishly, and I could tell that they had been planning something.