Scratch

Is there any tutorial or literature to read on processing between sprites?  I wanted to write all of the logic on my stage and reference the other sprites but apparently I don't see how to reference a sprite and take action on it from the main stage coding area.  So basically I wrote a program with my son which uses loops in the main stage and also in the sprites.  The loops share global variables between them and act on them inside the loops. 

Programming like this is not so straightsforward.  There is no GoEvents statements as there is in VB6 to tell the OS to stop executing a loop temporarily and let the OS handle any user initiated events (e.g., from the other sprites).   Without this statement, how do forever loops in multiple sprites interact?  Is each loop guaranteed to execute at a single point in time - in other words, am I guaranteed the the main loop will only execute once and the sprite loops not execute two or three times before the main loop gets to the bottom of the loop.

Scratch is a great program to learn programming without having to learn an underlying language but there are still some fundamental rules that need going over: e.g.,
- Can multiple sprites have a loop that starts when green flagged pressed?  What are recommendations?
- Can a loop be momentarily stopped by the user to allow the OS to execute a loop from another sprite to allow it to set data before the loop in question continues to execute?
- Can code from another sprite be called from the stage for example?
Thanks