Illogical English #3

Here is another one: An alarm
When an alarm goes on, it goes off…
In this case “on” means “switched on” (like all electrical things), but “off” means “start”. For example, when leaving somewhere, someone may say, “Right, I’m off.”
So it can be confusing in sentences like this: “The alarm went off at 11pm, and it didn’t go off until 11:30.”