Every battery actually holds a chemistry experiment! The first modern battery was created in 1800 CE by the Italian scientist Alessandro Volta (the "volt" is named after him). He realized that, if he stacked two different metals (copper and zinc in this case) separated by layers of cardboard soaked in salt water, electrons would start to flow through the whole pile, spontaneously generating an electrical current. Archaeologists may have discovered some even older batteries near Baghdad, Iraq, which date back to 200BCE.
Inside a battery, there are two metal electrodes surrounded by a sort of paste that lets the electrons flow. You might have noticed that most batteries are marked with a "+" (positive) and a "-" (negative) end. When the battery is being used, the negatively charged electrons flow from the negative end (called the cathode) to the positive end (the anode) - remember that opposites attract! Around and around they go, but eventually they run down, the electrons stop flowing, and you have a dead battery. Some batteries have to be thrown away when they die, but others can be recharged.
The next post will show you how you can make your own battery out of some things you probably have around the house. You'll also be able to see what effect varying the metals electrodes has on the voltage produced by the battery.