little confusion in analog stuffs

Code ( (Unknown Language)):

1. void setup()
2. {
3.   Serial.begin(9600);
4.  }
5. void loop()
6. {
7.   int a= analogRead(3);
8.   Serial.println(a);
9.   delay(1000);
10. }

but i have not connected anything to that analog pin but i am getting values around 500.whats the reason for that

 

Hard to say exactly why it could be say noise pickup. But a thumb rule in electronics is. Undefined input will get you undefined output. So measuring some floating input (not connected) do not give any meaning at all. It is like a radio tuned between to stations