In the movie Harry Potter and the Half Blood Prince, Harry looks at one of Dumbledore's memories of Tom Riddle. In the memory, Dumbledore's meets Tom and askes him to come to Hogwarts. When he walks into his room, it shows some odd objects. There are broken eggshells lined up on his windowsill, and a picture of the location of the locket (one of the horcruxes) in the doorknob. What do these object mean and why does Tom keep them? Thanks!