Ok so that's what that plastic ring is for! 

Gourmandize

Have you ever wondered why virtually all plastic bottles have a plastic ring below the lid, which you have to break open?

Well, it turns out that the plastic ring is essential for storage, transportation, and maintaining taste. It is commonly refered to as the "tamper evident band" and as the name suggests, it also shows if a bottle has previously been opened.

In some countries, it is common practice to refill old bottles of mineral water with tap water, and reseal the ring with glue. 

Pay close attention to this seal and if it looks broken or damaged, don't buy the bottle!