Metal Reaction Labby Thingy

Picture taken from my iPad.

To the left you'll see a photograph.  This might be meaningless to you but I shall try to shed some light upon the situation.
In this lab we tested the reactivity of metals with different substances.  We tested zinc, magnesium, and copper, shown respectively in the photograph.  In the first row we placed about 8 drops of copper nitrate.  The second row received magnesium nitrate.  The third row contains zinc nitrate.  And the final row contains silver nitrate.
All of the metals reacted with silver nitrate.  Silver nitrate was the only substance that the copper reacted with.  The magnesium reacted with the copper nitrate as well as the silver nitrate.  And, as you can see, zinc reacted only with silver nitrate and copper nitrate.
It's interesting that copper only reacted with silver nitrate.  What's also interesting is that the zinc reacted to the copper nitrate but the copper didn't react to the zinc nitrate.  I have no explanation for this.  I didn't really focus past, oh hey the metal turned black!  I'm sure the reasons will be clearer in the future.