Electric charge on rubbed objects

[IAmUnique]

When two objects say glass, fur, plastic, ebonite rod etc are rubbed with each other then why is it that glass rod, wool, fur gain positive charge while ebonite rod, plastic rod gain negative charge? I know that it's because of transfer of electrons...but why does the transfer of electrons happen in a specific way itself?

 

[Doug Huffman]

The electrons are more or less tightly bound in the various materials, most tightly bound in an insulator.