well, there are other techniques like writing with lemon juice and when it dries you can't see it unless you put it near the fire. I think it is like that...

That is all nice, but do you think soldiers in the war had lemon at their disposal? 😀 Not even those had lemon, who were at home and not in the front line.

so you have practiced fencing?

Yup, for a couple of years.