Why do hot water bottles have horiztontal lines on them?

Question

Why do hot water bottles have horiztontal lines on them?

Answer 1

By “horizontal lines“, I assume you mean the rubber “ribbing” which is on some (but not all) hot water bottles. This is simply to create a gap between the hottest parts of the bottle making it safer to touch if the water inside is too hot.

I prefer a smooth bottle myself. I simply make sure it is not too hot by filling it directly from the hot tap rather than a kettle.

Answer 2

Don't fill it past that point.  I disobeyed it once.  I was confined to bed once and put it under my back .  The thing blew its brains out and soaked the mattress.

-MM