Have you seen the news reports about teens playing the "Salt & Ice" game? It involves sprinkling table salt on your skin, and then putting an ice cube on top of the salt. You then are supposed to press down on the cube until the pain is too much to bear.
What's scary about this is that serious tissue damage can result. Frostbite is a real possibility. With frostbite, "ice crystals form in the space outside of the cells. Water is lost from the cell's interior, and dehydration promotes the destruction of the cell."*
Furthermore, "as blood flow returns to the extremities upon rewarming, it finds that the blood vessels themselves are injured, also by the cold. Holes appear in vessel walls and blood leaks out into the tissues. Flow is impeded and turbulent and small clots form in the smallest vessels of the extremities. Because of these blood flow problems, complicated interactions occur, and inflammation causes further tissue damage. This injury is the primary determinant of the amount of tissue damage that occurs in the end."*
Now that I've dealt with the biology, let's discuss the physics.
You learn in your science class that water freezes at 32ºF (0ºC). Now that's really only true for pure water. Dissolving any substance (salt, sugar, alcohol, antifreeze) in water will lower the temperature at which the solution freezes. A solution that is 23.3% salt by weight will freeze at -6ºF (-21ºC).
Here's what happens in the "Salt & Ice" challenge. The ice cube coming from a typical freezer may be at a temperature of 0ºF (-18ºC). Since it's in contact with your skin, some ice melts. The water that results is at 32ºF (0ºC), but it mixes with the salt. Since the salt-water is in contact with the ice cube, the water cools down to the temperature of the cube.
Let me repeat that last point. The salt-water is at a temperature as low as 0ºF (-18ºC).
It's the cold water that poses the danger here. It takes a relatively large amount of energy to change water's temperature. Scientist call this thermal property the specific heat. So energy is being transferred from your skin to the salt-water, your skin cools drastically, and the salt water barely changes temperature. In fact, the salt-water really doesn't change temperature at all, since it's still in contact with the ice cube.
We can use this physics we have have friends over for grilled meat. If you happen to forget to cool off the beer, add the drinks and ice to the cooler. Then pour in some water and some salt. You'll have cold beer in no time. The only way to get it colder and faster is with liquid nitrogen, but that can be a little hard to come by.