Here's a hint: it's another part of her body. As in, it was once attached to her person elsewhere, only to be removed and tucked away inside her belly for safekeeping.
Give up? It's her skull. Her skull! In her stomach! How amazing is that? But we're getting ahead of ourselves.
The abdomen in question belongs to former "Mrs. Idaho" Jamie Hilton. Back in June, Hilton was involved in a devastating accident, and had to undergo emergency brain surgery. In the process, her doctors removed 25% of her skull so that her severely swollen brain would have room to return to its normal size. Fully intending to reattach the skull fragment later, Hilton's doctors stowed it away inside her stomach, where her body would keep it nourished until she was ready to have it reattached.
In the weeks ahead, Hilton recovered marvelously, all while missing a full quarter of her skull. She wore a helmet to help prevent further injury.
By mid-July, Hilton's swelling had subsided enough for doctors to remove the fragment from her abdomen and put it back in place with the aid of some titanium screws and plates. It's been over two months since her second surgery, and she seems to be doing incredibly well, telling the New York Post that the only lasting effect seems to have been a change in her tastebuds and the loss of her sense of smell.
As for her head, it's back to its normal shape, albeit with a pretty awesome-looking scar. And we're willing to bet it will be barely noticeable once her hair grows back in.
You can read more about Hilton's remarkable healing process over on her blog, where she and her husband have catalogued the entire ordeal. For more information on the surgery that Hilton had performed (decompressive hemicraniectomy), check out this post by Bradley Voit, who wrote about the procedure in detail when Congresswoman Gabby Giffords had it performed back in January 2011.