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?

In computer science, object composition (not to be confused with function composition) is a way and practice to combine simple objects or data types into more complex ones