The first occurrence.
And God saw the light, that it was good: and God divided the light from the darkness. Gen 1:4
The primitive root.
Strong’s H3588 כי kiy, a primitive particle indicating a causal relationship, “that, because, for, if, surely,” &c. The root letters are kaph + yud.
kaph כ, ך = the open palm, thus bend, open, allow, tame
yud י = the closed hand, thus work, throw, worship
The story: Out of the 22 letters of the Hebrew aleph-bet, the kaph and yud are the only letters indicating two different states of the same object: the open hand, and the closed hand. The open hand (kaph) changes to become the closed hand (yud,) i.e., movement from one state to another, a causal relationship.