How did the Rosetta Stone crack Egyptian hieroglyphs?

The Rosetta Stone cracked hieroglyphs because it says the same thing three times. It carries a decree from 196 BC written in hieroglyphs, in Demotic (a second Egyptian script) and in Ancient Greek, which scholars could already read. With only minor differences between the versions, the Greek became a crib sheet for the two unknown scripts.

The trick was names. Foreign names like Ptolemy could not be written with picture-ideas; they had to be spelled out by sound. Scholars first found them in the Demotic text, and in 1814 the British polymath Thomas Young, following a tip to look inside cartouches, which ought to contain Greek names, identified the sound-signs that spelled Ptolemaios in hieroglyphs.

The decisive step came from Jean-François Champollion, a young Frenchman obsessed with Coptic, the last stage of the Egyptian language. Comparing the names Ptolemaios and Kleopatra on another monument, he built an alphabet of sound-signs, then realized that these signs also spelled real Egyptian words: he read 'MS', the Coptic word for birth, next to a Greek mention of birthday celebrations. Legend has it he ran to his brother shouting 'Je tiens mon affaire!' and collapsed. Egyptian writing, he showed, mixed sound-signs and idea-signs.