Now, it could just be that your mother is unreasonable (always a possibility) but I must say, the issue here is confusing the HECK outta me.
As others have said, as long as you don't fill up the kettle or whatever while she's taking a shower, there should be no possible way she could even be able to tell you did so. And you're boiling it, which makes the issue even more confusing.
As others have said, as long as you don't fill up the kettle or whatever while she's taking a shower, there should be no possible way she could even be able to tell you did so. And you're boiling it, which makes the issue even more confusing.