Experts say where and how a bullet enters the brain can make all the difference.
An inch or two to the right and the bullet would have instantly killed Giffords.
Two days after getting shot in the head at point-blank range, no increase in swelling is excellent news.
"We also had CAT-scans this morning that showed that the swelling is not getting any worse," said Dr. Michael Lemole, chief of neurosurgery, University of Arizona.
When surgeons like Lemole enter the brain after a penetrating injury, the goal is to clean the wound, stop the bleeding and relieve any pressure in the brain.
For the next two weeks, doctors will monitor for infections. In the next two months doctors will look for any loss in brain function.
So far, Giffords is able to respond to commands like hand-squeezing and holding up two fingers.
"It implies that a lot of centers in the brain are working. Your ability to, for example, hear speech and then decipher it and put it into a cultural context," said Lemole.
Dr. Neil Martin, the chief of neurosurgery at Ronald Reagan UCLA Medical Center, says how well Giffords recovers may depend on whether she is right-handed or left-handed.
If she is right-handed, then she relies heavily on the left side of her brain. If she's holding up two fingers with her right hand, Martin says it sounds like she is doing very well.
"Because that would indicate that the extent of damage to the critical left side of the brain and those areas controlling movement in the arm and hand are intact," said Martin.
People who are ambidextrous use both sides of their brain. Experts say in the case of injury they have an advantage.
"Ambidextrous people often can have a better recovery from a brain injury, from trauma or a stroke than people who are strongly dominant to one hemisphere, if that hemisphere is damaged," said Martin.
Doctors say Giffords isn't out of the woods yet. That's because the swelling in her brain can take three to five days to maximize. But every day that there is no increase gives them reason to be optimistic about her recovery.
Doctors will focus on monitoring Giffords for swelling in the brain for the next few days. They say it's too early to give a prognosis but the signs she's showed so far are remarkable.
UCLA'S Dr. Neil Martin says a closed head injury can hurt the brain just like a penetrating one.
"Areas of the brain can be bruised by impact of the brain with the skull, and that can cause just as severe a local injury to a critical control center," said Martin.
The brain can be retrained due to the plasticity of the brain.
"Uninjured areas of the brain can compensate for the functions that are lost by the damaged areas," said Martin.
Doctors used to believe once you lose brain cells, they never came back. Now new research has revealed something called "neurogenesis."
"Now we know that certain areas of the brain generate new cells throughout life, even in in adults," said Martin. "You can actually increase the level of neurogenesis in your brain, and one of the ways that's done is through active exercise, through aerobic exercise."
It'll take months to for doctors to figure out how much damage the bullet did to Giffords' brain, and it may take years for her to fully recover.