Java Programming

In the following code snippet , when i run it throws following exception :-

Exception in thread "main" java.lang.IllegalThreadStateException
	at java.lang.Thread.start(Unknown Source)
	at com.collections.Threads4.go(SCJP.java:323)
	at com.collections.Threads4.main(SCJP.java:312)

class Threads4 {
	public static void main (String[] args) {
		new Threads4().go();
	}
	
	public void go() {
		Runnable r = new Runnable() {
			public void run() {
				System.out.print("foo");
			}
		};
		Thread t = new Thread(r);
		t.start();
		t.start();
	}
}

Can anyone let me know why does this happen ?