How do Cu2+ ions oxidize Zn in a voltaic cell if separated?

[coreluccio]

Alright, so I understand how it works, except that, I don't get how zinc metal reacts with aqueous copper (II) ions if they are completely separated by two jars. How can a copper (II) ion pull the electrons from solid zinc when it can't even physically collide with zinc?

 

[zhermes]

They have to be connected by a salt-bridge, and also another (usually) wire to complete the circuit.