While staying as Hades’s guest, Core fasts. She refuses to eat for 10 days. At this next bit the myth changes based on who is telling it. Some myths say that she broke down eating 7 pomegranate seeds. Others say 6, still others say 3. One myth even says that she didn’t eat any seeds, but that Hades’s gardener swore that she did. This is important as it becomes part of the deal for her release. See, technically, eating something from the underworld makes her part of the underworld.
This isn’t said until after she eats (or doesn’t eat) the pomegranate seeds, but it would explain why she is fasting. As the daughter of Demeter (the aforementioned goddess of agriculture), she would understand the significance of eating the seeds, which would explain the fasting. Some might write this off as her sorrow for being taken from her mother, but it doesn’t explain everything, nor does it explain why she waited until she heard of her rescue (in the versions where she at the seeds).
The version of the myth where the gardener is lying would make sense in order to trap her there as part of the deal (which we’ll get into next week). However, Core doesn’t call him out on it. This would be a simple case of he said/she said, but she doesn’t say anything, which is odd. As the daughter of Demeter, her word would certainly carry more weight than that of one of Hades’s servants, yet she never brings it up, either because she really did eat the seeds or because the idea fit into her plans.