insulator可唔可以被earth?

The phenomenon of attracting pieces of paper by a charged (plastic) ruler is because of electrostatic induction.
In the actual fact, insulators are just bad conductors. You could earth any object as you wish. The only difference is that charges (or electrons) in good conductors can move much more easily and freely than those in bad conductors.
Therefore, once any charges are induced on the aluminium foil, they will flow away instantaneously. But for a bad conductor, e.g. paper pieces, charges could maintain for quite some time before they are lost.

翻譯 physics8801的答案：
唔怪 earthing 事，係因為 insulator 同 conductor的分子結構唔同，
conductor有一層 sea of electrons, 所以一induce到 charge, D charge 就游左去第度，成個object無polarised;
但 insulator, 或者話 bad conductor, 就無呢個特性，所以會被polarised, 產生吸引的現象。