You can have
several programs open at a time, but you can only work with
the frontmost or "active" program. To see the
programs that are open:
There
are two ways to change the "active" program (switch
between programs):
If
you have programs open that you are not using, you should
quit those programs by making them active and then choosing
Quit from the File menu. Doing this will help
you avoid getting low memory error messages.