Multithreading question C++

#include <stdio.h>
#include <windows.h>
#include <process.h>     // needed for _beginthread()

void  silly( void * );   // function prototype

int main()
{
    // Our program's first thread starts in the main() function.

    printf( "Now in the main() function.\n" );

    // Let's now create our second thread and ask it to start
    // in the silly() function.


    _beginthread( silly, 0, (void*)12 );
    // _beginthread( silly, 0, (void*)13 ); if I uncomment this I don't see 13 on output :(

    // From here on there are two separate threads executing
    // our one program.

    // This main thread can call the silly() function if it wants to.

    silly( (void*)-5 );
    Sleep( 1000 );
}

void  silly( void *arg )
{
    printf( "The silly() function was passed %d\n", (INT_PTR)arg ) ;
}

_beginthread( silly, 0, (void*)13 );