A tax decree cracked the hieroglyphs.
In 1799, French soldiers near Rosetta in the Nile Delta found a granodiorite slab bearing the same priestly decree of 196 BC in three scripts: hieroglyphs, Demotic and Ancient Greek. Because Greek could be read, the rest could be compared against it. Jean-François Champollion achieved the decipherment in 1822. The stone has been in the British Museum since 1802.
Useful: A known text beside an unknown one is the best key you can have, in languages as in data.
Source: en.wikipedia.org