People were able to read the Rosetta Stone because one of the languages on it was one that was well-known to scholars of the time. Because the stone had the same message written in three languages, understanding one them allowed scholars to have a good chance of deciphering the others.
Before the stone was found, scholars could not read Ancient Egyptian hieroglyphics. They also could not read Demotic script, though this was less important to scholars. These were two of the three languages on the stone. The scholars could, however, read Ancient Greek. This was the third language on the stone. They read the inscription in Greek and then used that knowledge to help them decipher the meanings of the other two scripts. This was not completely easy and actually took some years, but it was eventually possible.
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