As Father Guido Sarducci used to remind us, everything you remember from college can be taught in 5 minutes. 

And he wasn't kidding.

Within those 5 minutes are little sparks of knowledge.  They originate from a good teacher who imparted upon us something so simple yet profound that it made an indelible mark on our memory, causing us to never forget it.

One of these sparks I recall from law school came from a professor who taught his students how to identify when a tax occurs: "A tax occurs on a transfer of money.  Picture a government hand.  There are times when a buyer transfers money to a seller, interrupted by the government hand grabbing the money in the middle, taking out its share then handing the balance to the seller."

This definition of a tax occurring only on a transfer of money has served me well because the definition has never failed me.

A review of Black's