How did Cain and Abel know to offer sacrifices before the Law of Moses?

Genesis 4 has always bothered me. The story of God accepting Abel's sacrifice and not Cain's has led me to wonder: *How did they even know to sacrifice?* Was this practice started by Adam? Or did God establish more of a law than just 'Don't eat of the tree of the knowledge of good and evil'? If God did establish further law then what was it and when did He establish it?