@Mr Wolf: I didn't wrote they need, I wrote I did it that way. It is handy for me so the object remembers its parent. I merely suggested it as a good 'coding' practice.
In the even part the UID stands for any UID of the sprites object. They have to be uniqe so the picking is done right. Unique in the sens that it is right for only the ones you want to pick. You could pick more than one with such 'for each' loop.
I just thought I would elaborate on your hint as I reckoned it was to short and 3 days ago I wouldn't understand it myself.