How did Harry Potter know about the hufflepuff cup?

Hogwarts was the only place Tom Riddle (Voldemort) had ever felt truly at home, and shortly after arriving there, he became obsessed with his heritage. Dumbledore showed Harry a memory of Tom visiting Hepzibah Smith, who showed him two of the later horcruxes, one of Ravenclaw's (the diadem), and one of Hufflepuff's (the cup), which he later killed Miss Smith just for these two items. Harry and Dumbledore were able to deduce that Voldemort would look for other important objects relating to his heritage.