It's just paper and adhesive, but if you get the right old stamp in your hand, you might be surprised how much it can be worth. Rare stamps have sold for upwards of a million dollars at auction, and most old stamps are at least worth more than they their purchase price. So, if you are cleaning out your grandmother's attic and happen upon a box of old stamps, don't just toss them. Find out what they are worth.
Determine how old the stamp is. Many stamps have a date of issue printed on the bottom. You may need a magnifier to see it. Or else use the postmark, which will give you a general idea. Older stamps are rarer and will therefore be worth more.
Identify what you have. You can identify exactly what stamp you have by looking it up in a catalogue, such as Scott's Standard Catalog of Postage Stamps or the US Postal Service Catalog. You will need some information on the stamp to help you find it. Some idea of the year and the country of issue will be a big help. The catalogue will provide you a value for a stamp in mint or near mint condition for both unused and cancelled stamps.
Determine the stamp's condition. Whether the colouring is still vibrant or faded, the perforations and watermarks will all affect the stamp's worth. All of these things will lower the stamp's value.
Consult catalogues or guides to confirm value. Once you have an idea of the stamp that you have and the condition of the stamp you will need to consult a value guide or catalogue to confirm the value. You can also consult a stamp dealer. This will give you an accurate idea of the true value of your stamp.
Don't bother with recent stamps. If you want to look up every stamp you come across, go ahead, but the ones issued within the last ten years will be worth less than face value. Even those within the last twenty years won't be worth much more than they originally cost.