Electrons are transferred from one surface to another by physical contact, which is why it is important to rub the test materials with your hand or a piece of wool. By rubbing vigorously, you are transferring electrons from your hand or wool to the material you’re testing. Your hand and the wool become positively charged, while the test material becomes negatively charged. The electrons, which are negatively charged, can then travel through the copper wire of the electroscope down to the pieces of foil, which both become negatively charged. These negative charges push against each other, causing the pieces of foil to repel. Remember, while opposites attract, a negative-negative or positive-positive combinations of charges always repel!