One of the clues that enabled Western scholars in the 19th century to decipher Egyptian hieroglyphics came from the hypothesis that the ancient system of pictographs might resemble Chinese characters, in that at least some were intended to represent sounds rather than depict meanings.
That indeed turned out to be the case. And like hieroglyphs, a high percentage — perhaps as many as two-thirds — of the kanji in regular use incorporate easily recognizable phonetic components that give a pretty consistent indication of how they should be read.
Let's start with the character 古, read ko and also furui, both meaning "old," which is written with crossed lines above a mouth.