internal class Piston {} 

internal class Engine 
{ 
  private Piston[] myPistons = new Piston[4]; 

  public bool Ignition() { 

//some code 

  } 

} 

public class Car 
{ 

  private Engine myEngine = new Engine(); 

  public void Start() 
  { 

  //put in keys etc.. 
  if (myEngine.Ignition()) { 
    //some more code 
  } 
} 

What OO concept does the above sample code demonstrate?