A single curling stroke - pictograph of a young sprout pushing crookedly out of the soil, or a fish-hook's bend. As Kangxi radical #5 it heads a family of single-stroke twists:
九 nine,
也 also,
习 to practise. Borrowed early for the second of the ten Heavenly Stems, it now reads as 'B' in grading -
甲乙丙丁 'A-B-C-D'.