A single digit is a single sound, so the word's only job is to wrap that one consonant in free vowels (and h/w/y) to make it picturable.

Each digit maps to one consonant sound; the rest of the word is filler you can see. The ten words below are the classic single-digit pegs — concrete, instantly picturable nouns — chosen because vivid images are recalled far better than abstract ones. Learn them cold; they're the atoms the double-digit list is built from.

ace — the one coded sound is /s/ = 0.

The vowels are free; the c in "ace" is a soft /s/, which is a 0. Picture the ace of spades — the single card that beats the rest.