I am reading Modern Operating System (4th edition). I am not really sure about this paragraph.

> 
> Despite their better performance, user-level threads packages have some major
>  problems. First among these is the problem of how blocking system calls are implemented.
>  Suppose that a thread reads from the keyboard before any keys have
>  been hit. Letting the thread actually make the system call is unacceptable, since
>  this will stop all the threads.
> 
> 
> 

If a thread blocks waiting for an input, can't another thread be off doing something else? Why it would "stop all the threads"?