Understanding the Output from a microphone

Other Hardware Sensors
7

I connected the red wire to 5V, White wire to analog pin A0 and the shielding wire to ground. I've used the following code to read the values

const int MicrophonePin = A0;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);

void loop() {
  // put your main code here, to run repeatedly:
  int MicrophoneReading = analogRead(MicrophonePin);
  Serial.print("Microphone reading is ");
  Serial.println(MicrophoneReading);
  delay(200);

But the readings gradually increase. Anyone know why?