How a spark is produced? When we are closed to some metal sometimes spark is created. Is this because when we come close to a metal our charged fingers charge by induction the metal ? Then a voltage between our finger and metal is established so air gets ionized ? But where the positive and negative ions go? Negative to positive and positive to negative so metal and our finger become neutral ? Can someone explain this to me ? Thanks in advandance
 A: When we are close to some metal sometimes spark is created. Is this because when we come close to a metal our charged fingers charge by induction the metal ? Then a voltage between our finger and metal is established so air gets ionized ?
The spark is created by the breakdown (ionization) of air between your fingers and the metal object. The phenomenon is called electrostatic discharge (ESD). It is the result of a buildup of electrostatic charge on your body or the metal object. This build up can result in a high (thousands of volts), but generally harmless (because the potential current will be small), voltage difference between your fingers and the metal. The voltage can be high enough so that when your fingers come close to the metal object the voltage causes the air between them to ionize and break down. 
The build up of electrical charge can occur due to the three main mechanisms of friction, induction or conduction. Probably the most common involves friction. Friction involves rubbing two objects together causing electrons to transfer from one object to the other. Induction involves charging an object without actually touching the object to any other charged object. The electric field of a charged object brought close to another object induces the free electrons in the second object to migrate away from or towards the surface of the second object (depending on whether the first object is positively or negatively charged). Charging by conduction involves the contact of a charged object to a neutral object. For example, a positively charged metal plate is made to contact a neutral metal sphere causing the neutral metal sphere to become positively charged. 
But where the positive and negative ions go? Negative to positive and positive to negative so metal and our finger become neutral ? Can someone explain this to me ?
The process for removing excess charge (positive or negative) is called grounding. Grounding removes excess charge on an object by means of the transfer of electrons between it and another object of substantially larger size. A ground can be viewed as an electron “sink” (reservoir) to absorb electrons.  In other words, the ground is capable of transferring electrons to or receiving electrons from a charged object in order to neutralize that object. 
Hope this helps.
