tooth, cog
歯 centers on a tooth or something tooth-like: a cog or gear tooth that interlocks and drives motion.
歯 is the modern simplified form of 齒. The older form is a pictograph of a mouth with teeth, later combined with 止 for sound. The modern form retains the 止 top and a simplified lower part representing teeth.
The top 止 looks like a row of teeth, and the lower part resembles a mouth with teeth inside. Picture a cog's teeth interlocking like a smile.
For シ, imagine a toothy grin: she -> シ, and her teeth are perfectly aligned like cogs.
tooth; teeth
dentistry
dentist
dental brushing; brushing one's teeth
toothbrush
tooth decay; decayed tooth; dental cavity; dental caries
gear; cogwheel
gums; gingiva
brake; drag; skid; pawl
front tooth; foretooth; incisor
false tooth; denture
tooth powder; toothpaste
back tooth; molar
fern (esp. species Gleichenia japonica); fernery
chewiness; firmness; feel of food while being chewed
row of teeth; set of teeth; dentition
molar (tooth); grinder; premolar
tooth extraction
to strike back at; to bite back; to turn on; to rise against; to oppose; to defy
protruding tooth; overbite; bucktooth