電絕緣體的問題

You should be aware that the physical world is not as perfect as you thought. In reality, there is no "perfect conductor" or "perfect insulator". By a "conducting object", we mean that the object has a very low resistance so that charges (or electrons) can flow in it without much resistance. Similarly, an "insulating object" means the object has a very high resistance such that the flow of charges in it is extremely difficult. It is because of these properties that we feel/observe an object "conducts" or "not conducts".

With the above understanding in mind, your questions are answered as follows:
Q: 但問題係點解charge 係 rod 果到唔會earthing
The charges on the rod do leak through your body to earth. As said before, the rod has a very high resistance. The leaking of charges (i.e. the flow of current) is extremely slow. Since you do the experiement usually within a short period of time (within an hour), the leakage of charge is not observed. However, if you hold the charged rod in a long enough time, you could probably observe that the charges on the rod is gradually diminishing.

Q: 當個人掂到條帶就會earthing 令個人感到觸電
點解我幫metal charge 果陣條rod 唔會接地相反呢個會???
There is a constant supply of charges to the conveying belt in an escalator because of friction. The amount of charge residing on it is much much larger than the charges in the rod you used in your experiment. This sets up a high high potential on the belt. When you touch the belt with your bare hand, the high potential difference between the charged belt and earth (your body) causes a minute current to flow. (remember that by Ohm's Law, I = V/R, a high value of V over a high value of R still would give a small value for I). This is why you feel the electric shock.

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2010-03-06 13:34:37 補充：
Q: 點解2件野摩擦果陣 有電子流動??
This phenomenon is known as the "triboelectric effect". There are materials that electrons in them are weakly bounded, and there are materials in which their atoms are having spasely filled outer shell that tends to gain eelctrons....

2010-03-06 13:35:03 補充：
(contd)... When these two kinds of materials are in contact, electrons can be exchanged between them. Rubbing just help accelerating the electron exchange process.