The Wall That Spoke — how did we learn to read Egyptian hieroglyphs?

Hieroglyphs were cracked by finding one name somebody already knew. Each sign stands for a sound, so the signs inside a known royal name hand you an alphabet — and that alphabet reads other names nobody has ever translated. In a lamplit dig house you take the sounds off the Ptolemy cartouche from the Rosetta Stone, watch them light up in a second ring on the wall, and read Cleopatra without being told what it says. Then two broken chips teach the rule everybody misses: Egyptian runs left to right or right to left, and the animals always face the start of the line.