Vietnam War POWs Communicated by Tapping
Prisoners of war tend to have a language barrier with their captors, so it's par for the course that guards in POW camps will try to prevent their prisoners from speaking to each other, since you just can't tell whether they're complimenting each other's haircuts or discussing which guard to shank first. This is what happened to American prisoners during the Vietnam War, so the prisoners had to come up with a way to talk without even letting their captors realize that they were talking at all.
"HEY, MIKE! OUR ESCAPE ATTEMPT IS TONIGHT AT SIX! SCREAM BACK IF YOU UNDERSTAND!"
How Did They Do It?
So they came up with a tap code. The basic idea is that you tap out your message using a specific pattern. It's kind of like Morse code, only simpler, because you can't make a dash sound by tapping your fingers.
The tap code works by putting all of the letters of the alphabet on a 5-by-5 grid. (Well, 25 of them -- to make it fit, they had to cut out "K" and use "C" instead.) For each letter, you first tap the row and then the column -- so to tap out the letter "H," you'd tap twice for the row, then three more times for the letter. There's a decoder here, but unfortunately the prisoners didn't have access to one of these, so they had to do it all mentally.
And thus was born the first troll, tapping out "dongs" over and over again throughout the night.
Of course, once you're condemned to silence, it's kind of impossible to teach a code to someone unless, you know, they already understand the code you're using. So, as soon as his team was captured and he realized that they were about to be separated, Captain Carlyle "Smitty" Harris recalled being taught about the tap code by an Air Force instructor in his youth and quickly taught it to the three guys next to him, who in turn taught it to every prisoner they came into contact with. The result was that "the building sounded like a den of runaway woodpeckers," all of whom were tapping out carefully coded fuck yous to the Vietnamese.