#include "TwoWire.h"

void setup()
{
  Wire.begin(); // join i2c bus (address optional for master)
}

byte x = 0;

void loop()
{
  Wire.beginTransmission(4); // transmit to device #4
  Wire.send("x is ");        // sends five bytes
  Wire.send(x);              // sends one byte  
  Wire.endTransmission();    // stop transmitting

  x++;
  delay(500);
}

#include "TwoWire.h"

void setup()
{
  Wire.begin(2);                // join i2c bus with address #2
  Wire.onRequest(requestEvent); // register event
}

void loop()
{
  delay(100);
}

// function that executes whenever data is requested by master
// this function is registered as an event, see setup()
void requestEvent()
{
  Wire.send("hello "); // respond with message of 6 bytes
                       // as expected by master
}