But now the same reasoning applies to the stick. You know that sticks can't move themselves, so yet a third thing must be moving the stick (which is in turn moving the stone). And again, you do see that something is attached to the stick: someone's hand. So now you have your explanation for the movement of the stone. A human being, who does have the power of self movement, unlike the stick and the stone, is what is pushing the stick which is pushing the stone.
Hold that thought...
Now, consider what is meant by "change." When something changes, it goes from state X to state Y. Some water in an ice cube tray goes from liquid to solid. When it was liquid, it was actually liquid but potentially solid. And after it freezes, it is actually solid and potentially liquid.
Since a potential is simply a future state that something could change into, a potential state does not actually exist (yet) and so cannot cause anything. When the water is still liquid, that is to say, when it is potential ice but not actual ice, it can't cause your drink to get nice and frosty cold. Only actual ice can do that.
So a potential future state cannot cause anything, which includes making itself actual. In other words, the future ice in your ice cube tray cannot cause the water in the ice cube tray to become solid. A future state (a potential) cannot cause anything; only a current state (actual) can cause anything.
So a potential cannot make itself actual.
Now remember the hand-stick-stone example from above. The stone can't move itself, so if it's moving, something else must be moving it.
Similarly, if something is changing, it can't change itself and so something else must be changing it. That is to say, a potential cannot make itself actual, so if it is becoming actual, something else actual must be causing it.
In the case of water in your ice cube tray, it would be the cold air in the freezer.
But if the same thing applies to that thing, then like with the stick above, something else must be changing it, and so on.
In the case of the ice, the freezer compressor is causing the air to be cold. But something else is causing the freezer compressor to work: the electricity. And so on.
And like with the stone example, on the other end of the chain you must find something that is capable of causing change without itself needing to be changed by anything else.
That would have to be something that is just actual, with no potentials at all.
Because, if it had a mixture of actual and potential, then it would need something else to change it and would not be the "hand" that we need to explain the movement of the whole chain.
So the conclusion of the argument is: there must be something purely actual that is driving all change (and hence all events and activities) in the universe.
What is something purely actual like? Stay tuned...