<prompt> Human: So, a friend of mine told me he just finished paying off 5 years of back taxes, and that the IRS tacked interest onto it. Why do they do this? What is their reasoning? Its not a loan. Am I missing something obvious here? Is this common?  <response>: Moneys due by tax day. If you take longer, the IRS basically is giving you a loan involuntarily, I might add, so you owe interest. 