It is important to understand Twain's "Advice to Youth" as a piece of satire. He isn't actually encouraging them to lie, be dishonest, and disobey their parents. He's simply parodying the kind of stern moral lectures that adults frequently dish out to children but turning the advice they normally give on its head.

So where an adult would normally tell children that lying is wrong, Twain tells his audience that they should be careful about lying, otherwise they might get caught. Instead of refraining from lying, young people should be cautious in deploying this great art until such time as they have the experience and the confidence to make lying both graceful and profitable. Here, Twain is satirizing the dishonest behavior of the very adults who presume to lecture children on good behavior.

Indeed, lying is such an intrinsic part of the adult world that, according to Twain, the truth isn't hard to kill. Far from truth being mighty, it is weak, as the most cursory glance at history tells us. History is jam-packed with examples that truth isn't hard to kill, and that, by contrast, a well-told lie can become immortal.

Far from avoiding lies completely, Twain's young audience should only avoid the awkward, feeble, leaky ones, the lies that cannot endure. If they want to tell enduring lies—like the adults who presume to teach them the rudiments of good behavior—then they need to start practicing how to lie as soon as possible. Only then will they be able to go on to construct those enduring, immortal lies that are strewn throughout history, and which form the basis of life in the adult world.