Re: My boss was demonstrating the instrusion sensors on our building
Well, there is a pretty easy way to remember the difference between "brought" and "bought".
"Brought" is the past tense of "bring". "Bought" is the past tense of "buy".
Both BRought and BRing both start with a BR. If you've had to BRing it, it's something you've BRought (or, er, BRung).
If it's something you had to BUy, its past tense doesn't have a BR either, therefore "BOUght".