Hot water bottles: Why the ribs aren't just for gripHow to get two different temperatures from one bottle.

From the show

Why it looks like that

The ridges on a hot water bottle are a clever safety and efficiency feature. By creating air pockets, they reduce direct skin contact, allowing for a gentler transfer of heat and lowering the risk of burns. The increased surface area also helps the bottle release warmth more slowly and evenly. Some bottles have a flat side for high-intensity heat and a ribbed side for milder warmth. It's a built-in temperature control system that most of us overlook.