Consider a stamp to be a receipt for payment to the post office for the service of mail delivery. By affixing it to the letter, the post office knows you paid and will deliver it. Letters without postage are called "undeliverable," meaning that no one is able to take them, so they will be returned to the sender. Stamps are sold in quantities of sheets or pads. Sheets of stamps have two sides with different images or messages. Pads of stamps contain eight or nine sections, each section containing an image or message on both sides. The cost of stamps varies by denomination and can range from less than 1 cent to more than $10. Most countries' postal systems provide for letters to be delivered only once. If there is no response, then another letter must be sent. This is called "third-class" mail because it costs only half as much as first-class mail. Some countries also have "second-class" mail which is even cheaper than third-class mail; however, in these countries, letters receive a reply only if you give your address directly. Otherwise, they are returned to you. First-class letters and packages come with a special label to indicate that they are not being sent through the normal post channels but rather through the priority system.