electrochemistry - Why do electrons follow from Zn to Cu but not Cu to Zn in the lemon battery experiment?

In the lemon battery experiment, as I understand, both $\ce{Zn}$ and $\ce{Cu}$ meet acid in the lemon, cause $\ce{Zn}$ and $\ce{Cu}$ release electrons to form $\ce{Zn^{2+}}$ and $\ce{Cu^{2+}}$.

So why do the electrons follow from $\ce{Zn}$ to $\ce{Cu}$ but not from $\ce{Cu}$ to $\ce{Zn}$?