Any material is made of an agglomeration of protons and electrons arranged in particular ways. Because like charges repel and unlike charges attract, and any particular point on the surface is going to be closer to some particles than to others, materials may end up with a tendency to attract or repel electrons: cotton shows no such tendency, while human skin repels electrons quite strongly and silicon attracts them quite strongly. Here is part of the triboelectric series, which lists one or two materials, with those at the top tending to repel electrons and those at the bottom tending to attract them.

Human skin

Rabbit fur

Glass

Human hair

Nylon

Wool

Cat fur

Silk

Aluminium

----Cotton----

Wood

Rubber

Copper

Sulphur

Acetate

Rayon

Polythene

Silicon

(Notice that metals and non-metals appear in the lists both above and below cotton; so there's no obvious, simple reason why the list should be as it is.)

• If you put two dissimilar materials into contact (it may need to be quite close contact) then electrons will, in general, transfer from one to the other.
• If the materials are insulators then the electrons, not being able to travel, will remain in their new situation.
• If you then separate the materials, the migrant electrons may remain trapped.
• If you then bring two new parts of the materials into contact, the process can repeat itself.

Rubbing polythene with your hand, for example, will satisfy all four bullet points and result in a significant charge build up because the two materials are a long way apart in the triboelectric series. Here is a diagram showing how easy it is to charge a polythene strip cut from a polythene bag and folded in half and thereby to convince yourself that like charges do indeed repel (on the grounds that, having been treated in identical fashion, the strips must have the same of whatever it is). The series tells you that rubbing anything with your hands will leave the object negatively charged (as long as it's an insulator so that the charge doesn't leak away).

I got the list off the internet, but I don't believe the position of acetate, which I think comes higher up than silk. The reason I think this is because the way we produce positive charge in the lab is by rubbing acetate with silk, which seems to leave the acetate positively charged.

The idea seems to be that when the two outside atoms of the two materials' molecules come into contact, they bond chemically, which means that an electron from one or other either transfers across or gets shared. When the two materials separate again, the electron gets trapped on the foreign body. Received wisdom is that only electrons can do this; but it seems to me that when the two materials are organic, with huge numbers of hydrogen atoms on the outside, that there exists the possibility for protons to transfer, rather as they do in the case of the hydroxonium ion. A kind of hydrogen bond, which then gets severed on the 'wrong' side? I don't know what the evidence is for its only being electrons.