Young boys in the Civil War era would write "18" on the soles of their shoes to verify their age so they could enlist. Cannot remember where I heard this or the details. I saw an old photo of a boy stretched out on the ground showing the number on the soles of his shoes. More info?
During the war the regulations specified that men had to be over 18 years of age to enlist. Because at the time there was just about no way to actually verify someones age, the enlistment officer would have to take the recruits word. Because a mans word was seen as such an important thing at the time lying was particularly looked down upon. To get around this recruits who were younger than 18 would write the number 18 on the the soles of their shoes, or on a piece of paper and then stand on it. I have also heard of planks being used. What this allowed them to do was respond to the question of their age with something along the lines of "I'm am over 18" because they were in fact, literally, over 18. So technically they weren't lying. How much this actually happened I have no idea, though I suspect its one of those things that has been exaggerated over the years.
Here is an article from civilwar.org that mentions the practice, if you could call it that.