Referencing two instance of the same object type

I'm sure there is an easy way to do this but I haven't seen it and not been able to find it in my searching unfortunately. I'm looking to reference two of the same object.

So for example lets say I have created 10 instances of an object, I want to loop through those 10 objects and changes a variable on each of them but reference another instance of the object. So lets say we do something like this (writing in Sudo code)

On Ball Collision (We'll call this #1)

    For Each Ball (We'll call this #2)

        Ball#2.height = Ball#2.height - Ball#1.height

You can see I'm trying to reference 2 objects which are both called ball, but not sure how I would be able to do this. Another example of this would be I had several buckets and I wanted to move the contents of the buckets down the line

For Each Bucket (#1)

    For Each Bucket (#2)

        If Bucket#2.id = Bucket#1.id + 1

             Bucket#1.contents = Bucket#2.contents

A way around this would be to store an the instance of the object in a variable. When I try and search for this all I'm able to find is information on instance variables, and while they are helpful they aren't exactly what I'm looking for ;)

I hope this makes sense, and hopefully someone will be able to help me out with this. Any help I'd be able to get would be greatly appreciated!

Here is an example that involves handling collisions between two instances of same object:

Evil sprites demo

So it looks like the best way to do things is to use "pick instance" which is effectively what I was looking for. I can see some situations where that might a little tricky still, but I think in most cases I was looking for this should help. Thanks for that!

For people who search for this you can find more information on it here:-

scirra.com/manual/124/system-conditions

It's a system condition. Another system I'm thinking of playing with storing the UIDs in an array and then using this to reference the different objects.