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1. Secret marks!
Some stamps have secret marks that are often difficult to see. They are called watermarks and are used to try to stop people copying the stamp.
There can be two identical stamps but with different watermarks. The watermark can affect the value of the stamp.
Watermarks have appeared on British stamps from 1840 (when the first stamp was issued) to 1967. Some countries still produce stamps with watermarks.
British watermark designs:
Next page you'll find out how stamp collectors find the watermark