When you rub two different materials, electrons (tiny negatively charged particles) get yanked from one material to the other.
This transfer happens because some materials have a stronger attraction for electrons than others.
The material that loses electrons ends up with a positive charge, while the one gaining electrons becomes negatively charged.
Not all materials can be charged by friction. Metals, for example, conduct electricity too well. Their electrons move freely, and any charge gets spread around easily, preventing an imbalance.

The best materials for friction charging are insulators, like plastic, rubber, fur, or glass. These materials hold onto their electrons more tightly, allowing a charge buildup when rubbed against another material.
The classic example is rubbing a balloon on your hair. The balloon (usually plastic) steals electrons from your hair, making the balloon negatively charged and your hair positively charged. This is why your hair stands on end!
Similarly, rubbing a glass rod with silk cloth can make the glass positively charged and the silk negatively charged.