Ashley was five years old the day she decided to put a straightened paper clip into an electrical outlet. This resulted in scars on her split-open fingers, burn marks in the carpet, and a memorable trip to the emergency room. Assuming they hit the hot prong, most children merely receive a painful shock when they do something like this (hitting the neutral or ground prong would have no effect at all in a properly wired system). It can be difficult for people sometimes to understand why electricity behaves differently in different situations.
The human body is very much like a thin rubber bag full of salt water. Usually the thin insulating membrane known as our skin does an excellent job of protecting us from all manner of harm, such as a poke with sharp stick, bacteria, viruses, dirt, and (only becoming an issue in the last 100 years) electricity.
If any of these things are able to get past the protective barrier of our skin, they can cause a great deal of harm. Cuts, sores, scrapes, abrasions, and rashes all represent weak spots in our skin and all of them are potential routes for infection. They are also potential routes for electric shocks to do much more harm than we are accustomed to them doing.
Although the skin has a very large resistance (on the order of MegaOhms), the salt water (electrolyte) solution that is in your body is designed specifically to conduct electricity, so that your nerve impulses can reach the muscles and do those little things we enjoy doing so much, such as walking and breathing. Normally, dry skin in good condition is insulating enough to provide fair protection against electric shocks up to 120VAC, such as is found in a typical North American electrical outlet. 240VAC is quite capable of knocking you flat on your ass and rendering you unconscious for a while, but still little lasting damage is usually done.
Of course, this is all assuming the skin is dry and in good condition. Breaks in the skin of the same type that can expose our delicate insides to infections can also expose our very conductive insides to electric shock. Once past the insulation of the skin, electricity has a free ride through the body. Two equations are helpful to put this into perspective:
I=V/R and P=IV
Current (I) equals voltage (V) divided by resistance (R)
power (P) equals current (I) times voltage (V).
Since skin resistance is high, current is usually low by I=V/R. Therefore the power coursing through the body is low by P=IV. However if there is a break in the skin, the electrical outlet does not see the high skin resistance, but rather the very, very low resistance of our muscles, blood, lymph, and internal organs, all soaking wet with very efficient electrolyte solutions passing nerve impulses from our brains to our organs.
In this case, since resistance is low, I=V/R means high current, and this implies P=IV means high power. Enough power to heat up a paper clip quickly and boil some of the water in the little fingers that are holding it, splitting them open. Enough power to burn two holes in the carpet as the electricity seeks a path to ground through those little fingers, down the body's highly conductive insides, and out what must have been her knee and foot on the carpet. Certainly enough power to send a five year old girl to the emergency room.
Another possibility is that her fingers could have been fine initially, but the outlet could have been damaged or faulty on the inside. If the paper clip was allowed to touch both the Hot and the Neutral somewhere inside the outlet (which should not be possible), the paper clip would have heated up hot enough and fast enough (like the nichrome wire in a toaster) to burn a hole through her skin, which resulted in the rest of the shock described above.
The real danger of electricity is that the power it represents is so dependant on a hundred variables that make up the resistance it sees. This means that the resistance value can be tricky to pin down, and it has a huge impact on how much damage a shock can do. A wet floor, wet fingers, a break in the skin, an arm resting on a conductive surface, the insulation value of your shoes, the length of body and specific path it has to travel through, whether or not it would cross the heart on its way to ground, and many others all play a role. Don't expect the circuit breaker to protect you. They trip at a whopping 15 Amps of current, where it takes less than one Amp to kill.
Even professional electricians occasionally forget this. It is a somewhat common, but very stupid, practice to test whether a wire is hot by touching it briefly with the fingers. As described above, if the skin is dry and unbroken and your shoes are well insulated, this will result in a mild, low current shock, but any "manly man" can take it. But what if your elbow is touching a metal conduit or junction box? What if it's hot that day and you're glistening with salty, conductive sweat from working hard? What if you grip the stranded wire too hard and one of the thin, needle-like wire strands pokes through your skin? A painful electrical burn can result if you're lucky. Permanent muscle damage can result if you're not. If you're really unlucky, it could pass through one hand and across your heart on its way out the other hand, and you could die. A million-to-one shot like this only has to happen once.
So the question is this: How can we prevent this from occurring? Well the obvious solution is to use those plastic outlet covers they sell at all hardware stores. These white, beige, or clear plastic covers have two plastic prongs which fit into the electrical outlet to cover it up and prevent small fingers from poking small metal objects into it. Another solution is to install a Ground Fault Circuit Interruptor to protect the outlet. These are safety devices (usually intended for wet locations) that detect when the electricity is flowing to ground (i.e. through a person's body) rather than through a light bulb and back into the outlet like it's supposed to be doing. They act very quickly and can significantly reduce the amount of damage done by an electric shock. I should mention that protection against poking paper clips into an outlet is not their intended use, but it would be better than nothing.
The problem with the outlet covers is that curious little fingers can pull them out. The problem with GFCI outlets is that they operate on a "better safe than sorry" philosophy and when they trip, you'll have to reset any clocks plugged into them and important things like the refrigerator and lights will lose power. This isn't an issue though if those items are on a different circuit, and only installing the GFCIs in locations where children might poke into them and in wet locations (their intended use) minimizes the nuisances they can cause.