Fill bottle one-third full with water, below top of copper tube ... depending on how clean the seal between the rubber stopper and the rocket is and how firmly the stopper is placed.

If you use it for longer, it could lead to severe scalds and burns if the stopper becomes loose or the rubber perishes. How to check whether your hot water bottle is out of date Now that you know ...

In fact, if your hot water bottle is more than two years ... also look out for split seams, perished rubber, and poorly fitting stoppers.” These are signs that the bottle may be unsafe.