Imagine that you're in your front yard, and you can hear your neighbor, Mr. Bixby, sawing something in his woodworking shop in his garage. Suddenly, he screams out in pain. You run into his garage and it doesn't look good -- Mr. Bixby accidentally cut off his finger. You stop the bleeding, call 911 and ice the finger. Because of your quick actions, surgeons are able to reattach Mr. Bixby's finger. But here's a question. Why did Mr. Bixby need a surgeon to reattach the finger? Why couldn't he simply grow a new finger? A salamander can regrow a whole limb that's been cut off – even Claire from "Heroes" can -- why can't we?

It turns out that salamanders are pretty unique. Yes, some lizards can regrow their tails. And humans can regrow fingertips in many cases, especially if a kid loses a fingertip. But, of all the thousands of vertebrates (animals having backbones) on the planet, including fish, amphibians, birds, reptiles and mammals, salamanders are the only ones that regenerate entire limbs. So, why can salamanders do it, but not human beings?

The way that salamanders regenerate a limb is really interesting. First, salamanders have a way to cut off blood flow automatically when they lose a limb. That way they don't bleed to death like many people do when losing a limb. Second, the cells on the surface of the salamander's stump go through a process that makes them look like the cells found in an embryo. When you were a fetus, inside your mother's womb, you started as one cell. Then you turned into a ball of cells. Eventually, there came a time when your limbs developed. All of your limbs grew using stem cells. A stem cell is a cell that can turn into any type of cell. So as your limbs developed, the stem cells turned into things like muscle, bone, skin and blood vessels. The cells all followed a "program" (that scientists don't completely understand yet) that mapped everything out.

In a salamander, the surface of a wound gets covered in stem-like cells. These cells seem to replay the same "program" that lets an embryo grow its limbs. In other words, a salamander regrows a limb the same way an embryo grows a limb. This lets a salamander regrow a leg once, or twice, or even a dozen times. In human beings and most other animals, this doesn't happen. In humans, the surface of the wound gets covered in cells that turn into scar tissue. Scientists are hoping that, over the next few years, they can figure out how to replace the scar tissue cells with limb-growing cells. Then humans will have the ability to regrow limbs too.

Meanwhile, other scientists are working on replacement organs. Let's say your heart goes bad. Scientists are working on a system that would let them grow you a new heart from your own cells. This technique also uses stem cells. Then a doctor could take out your old heart and put in the new one. It will probably take 10 or 20 years for all this research to reach the goal. But in the not too distant future, scientists hope they will be able to replace just about any body part.