Roll-on bottles can be made out of metal, glass or plastic, and they are most commonly used to hold perfume, deodorant or cosmetic products. After the product has run out you may want to remove the roll-on ball to reuse the bottle. Unfortunately, you can't pull out the ball with just your hands, as it is securely held in place to ensure that none of the product leaks. To remove the ball, you just need a few common items.
Gently hold the middle of the roll-on bottle with one hand. If the bottle still contains some liquid, you may want to hold it over a sink.
Squeeze the handles of the pliers so that the tips of the pliers grasp around the plastic base that holds the roll-on ball. While still gripping the base, slowly move the pliers from left to right to loosen up the base. When it feels loose enough carefully pull the pliers upwards to remove the plastic base and the roll-on ball.
Turn the roll-on ball upside-down so that you are looking through the bottom of the plastic base. Push the underside of the roll-on ball with the ballpoint pen until it pops out of the base.
If the roll-on ball is extremely small you can use an unfolded paper clip or pin to pop it out. The ball can be inserted back into the bottle base by pushing it through the underside of the base until it clicks back into place.
If the roll-on bottle is made out of glass be sure not to apply too much pressure, as it could cause it to shatter. Never try to remove the ball using your teeth -- it is dangerous, as the ball could pop out into the mouth or chip a tooth.