Multithreading _ join() method

public class Main implements Runnable
{
public void run()
{
System.out.println("r1 ");
System.out.println("r2 ");
}

public static void main( String[] args )
{
Thread t = new Thread(new Main());
t.start();
System.out.println("m1 ");
t.join();
System.out.println("m2 ");
}
}
why this error?
Main.java:15: error: unreported exception InterruptedException; must be caught or declared to be thrown
t.join();
^
1 error