again, twice, second time
再 centers on repetition: doing something again, a second time, or twice. It expresses the idea of a repeated action or occurrence.
再 is thought to represent a repeated action, possibly derived from a pictograph of a fish trap or a structure used again. The exact historical origin is uncertain, but the meaning of repetition is well established.
The top stroke 一 and the lower shape 冉 together look like a structure being built a second time: 再 means again or twice.
For サイ, imagine a sigh of repetition: you sigh (サイ) when you have to do something again. The sound of a sigh cues サイ while the kanji means again.
re-; again
restoration to life; coming to life again; resuscitation; regeneration
again; once more; a second time
reappearance; reemergence; return; revival
reopening; resumption; restarting
a second time; again; once more; twice
meeting again; reunion
rebuilding; reconstruction; rehabilitation
return; relapse; recurrence
second marriage; remarriage
reorganization; reorganisation; reshuffle
resale
return; coming back
recurrence; revival; resuscitation
re-election; reelection
another showing (of a play)
comeback; recovery; restoration; rally
reconsideration
again and again; repeatedly
retrial; reopening of a case
year after next
week after next
reappointment
month after next