starts a sentence about something that has just come to mind: "come to think of it", "that reminds me". Usually what was said or seen a moment ago set it off. It is mostly spoken and is often written in kana,
.
、きごう？
That reminds me, did you hear from him?
、。
Come to think of it, I ran into Tanaka yesterday.
、きごう？
Come to think of it, what happened with that matter?
The rest of the sentence sets the register; そう言えば fits casual and polite speech alike.
The た of remembering
A fact you have just remembered often goes into the past, even when it is about now or the future. does the same for a question about something you have half forgotten.
、。
Come to think of it, tomorrow's a holiday.
、きごう？
Come to think of it, what day is next week's meeting again?
そう言えば or ところで
switches topic on purpose: "by the way". そう言えば presents the new topic as something that just occurred to you, so it sounds less abrupt.
、きごう？
By the way, what are we doing about next month's trip?
As a reply, means "now that you mention it, you're right".
Lingaku