Liar Liar Pants on Fire

While writing a comment recently I realized how terribly lying can mess up a case.  Sure, some people stretch or omit some of the truth- but how many out there are straight up liars?  On the blog I was commenting on, a young boy lied about his identity to, in his words, "see Hollywood".  This may only apply to widely publicized cases, but there are some crazy people out there- how do we keep these liars out of the courtroom?  This statement may seem obvious, but a liar can't be trusted.  That being said, even if an attorney finds a liar that supports their case, how can they trust that this liar won't lie to them and ruin their case?  Perjury (the crime of a witness lying in court) is only a crime if the lie was a complete lie that pertained to the case.  If a witness lied about their birthday, for instance, they did not commit perjury.  But isn't this a prerequisite to lying about something pertinent to the case?  We have basic ideas (watch this it's pretty entertaining) to tell if someone is lying, but if those ideas are out there a person can fight back those instinctive lying actions and lie even better than before.  Like someone asked in class one day, in reference to attorneys who know their client is guilty, "Don't these people ever feel guilty?"  We can't rely on all people having a conscience.  This fact is the reason for most crimes and for many exhausting trials.