How does a Leyden Jar work? When electricity goes inside the Leyden jar, the inside of the jar is negatively charged. Then the aluminum foil on the outer layer of the jar is positively charged. Does the inside of the jar want to cancel out the aluminum foil, but it can't because of the insulator between the two conductors? 
 A: Practically, The Leyden jar is just a capacitor. The Leyden jar was originally used to store electric charge after some rubbed object is charged (like your socks in the dryer). The simplest capacitor contains two parallel metal plates with nothing in between them. If you add charge to one side of the plates, this will pull the opposite charge onto the other plate (assuming there is a path for the charge to get on there). 
          
This is what it would look like. In this example, there is a charge of +Q on one plate (and -Q on the other) with an electric potential difference of ΔV. The ratio of charge (on just one plate) to the potential difference is defined as the capacitance such that. The units for capacitance is the Farad.

However, it turns out that the value of the capacitance only depends on the physical configuration of the device. In this case that means the size of the plates, their seperation distance and the material that is in between them. For a parallel plate capacitor (like above) the capacitance can be calculated as:

The area of the capacitor is A and d is the distance between plates. The variable ε (epsilon) is called the permittivity and it depends on the type of material between the plates.
Even though the Leyden jar is in a different configuration, it basically works the same way. The outside metal can be grounded simply by holding it with your hand or running a wire to a metal water pipe. When you bring an charged object (like a plastic pen that your rub in your hair) near the metal in the middle, this will add charge to the water and draw the opposite charge to the outside metal. It's possible to get this up to fairly high voltage since the glass in between the water and metal acts as an insulator.
Coming back to your question,
When a charged object is placed on the metal conductor sticking outside of the jar, the electricity flows from the nail to the foil inside, while the foil outside is properly insulated from the nail. The outside foil gets its charge from the charged object you touch it with. This charge then pulls the opposite charge to the inside foil. Charge does not pass through the insulator - the electric field "pulls" charge to the plate on the other side (and that plate in turn "borrows" the charge from whatever else is attached, e.g. The mail in the cap...)
A: All you have in a Leyden jar are two thin tin foils on the interior and exterior of the jar.When you pass a current like you said one plate is going to have -q amount of charge and the other will have +q amount of charge.But they cant cancel out each other since the have a insulator in between that doesn't allow the charges flow. 
