IO Buffering...

I have a question on I/O buffering, can somebody help me out:

int main() {
        int c;
       
        printf("Hello World from PID=%d.",getpid());        
        sleep(10);
        printf("Hello Again, World! from PID=%d.\n",getpid());
        return 0;
}

the above program sleeps for 10 sec and then print out's the printf's.

Whereas, the one below print's out the statement before new line and sleeps for
10 seconds. I want to understand why is that ?

#include <stdio.h>

int main() {
        int c;
        printf("Hello World from PID=%d.\n",getpid());
        sleep(10);
        printf("Hello Again, World! from PID=%d.\n",getpid());
        return 0;
}

Thanks