Why is it possible to loosen screw top bottles by dipping the cap briefly in hot water?

The cap of a screw top bottle is usually made of plastic, while the bottle itself is made of glass or metal. When the cap is dipped in hot water, the plastic expands more than the glass or metal, causing the cap to loosen. This makes it easier to unscrew the cap.

The amount of expansion depends on the type of plastic used. Some plastics, such as polypropylene (PP), expand more than others, such as polyethylene (PE). The temperature of the water also affects the amount of expansion. The hotter the water, the more the plastic will expand.

In general, dipping the cap of a screw top bottle in hot water for a few seconds is enough to loosen it. However, if the cap is very tight, you may need to dip it in hot water for longer.