Raju was able to charge a plastic comb by rubbing it in his dry hair, but not a metal comb. Why?
Metals are a good conductor of electricity. They cannot hold static charges as charges keep flowing through them, our body and then ultimately down to earth. Plastic comb, on the other hand, is an insulator which can keep static charge build up. This is why it attracts pieces of paper and a metal wouldn't (as it has no static charges all have travelled down).