How do I pass a value from one macro to another? For example (simplified code)

sub aa()
mynum = 12
Application.Run "bb" ' pass the value of mynum to "bb"
' get the value of myvalue back from "bb"
'do some other stuff in "aa"
end Sub
----------------------------
Sub bb()
counter = 1
do while counter <= mynum
' code to execute in loop
counter = counter + 1
loop
myvalue = 66 ' pass the value of myvalue back to "aa"
end sub

Any help much appreciated, thanks, Brett