Unfortunately, I have no idea how to work with UID yet, so it was hard for me to follow your instructions.
So, I took a look on the manual but it didn't help me to understand exactly what I'm doing.
I did create an instance variable called: 'Selection' by clicking on the Sprite2 (the animated selection sprite) on the layout. but from that point you'll see what a mess I did because I need to understand it better, the manual didn't help me much because I better see see this in action to understand the logic behind it of store and restore variables and such.
Lets say I don't have Sprite2 at all, (I just made it for fun anyway), is it still possible to do? or it will be easier with this extra sprite?
Can please you try to instruct me more specific, step-by-step. I hope that I'll get more confidence in C2 by the day, I'm already messing with it a few days now and discover and learning new things every day!
Here is a screenshot of what I did (marked the lines).