What's the best way to fire someone?

[ElaineMonster]

You have to fire someone because they did something wrong. Do you tell them what they did wrong or do you just let them go without or with little explanation? Do you lie?

Example:
Employee was constantly tardy and missed too many work days.
Truth = I have to let you go because you were tardy too often and you missed too many work days.
No reason = I have to let you go. Please pack your things and leave the building now.
Lie = I have to let you go because we simply can't afford you right now.