Why does a ring stay on a PET bottle after you open it?
Short answer
The ring makes it easier to see whether the cap has been opened. Thin links, called bridges, connect it to the cap. They break when the cap is first unscrewed, leaving the ring below the bottle opening.
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1. Before opening
Bridges connect the cap to the ring. The ring catches under a ridge on the bottle neck.
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2. Turning the cap
The cap rises as you turn it. The ring is held by the ridge, so the bridges break.
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3. After opening
The bridges are broken. The cap comes off, while the ring stays on the bottle.
- Cap
- Ring
- Bridges
- Bottle neck
What happens when you turn the cap?
On many PET drink bottles, the cap has a main part, a lower ring, and small links between them. These links are called bridges. Inside the ring is a lip that catches under a raised ridge on the bottle neck.
As you unscrew the cap, its threads make the main part move upward. The ring is held in place by the ridge on the neck, so it cannot rise with the cap. The bridges are pulled until they break. The cap comes off, while the ring stays on the bottle.
What does the ring tell you?
Before the first opening, the bridges are intact. After the bottle has been opened, they are broken, giving you a visible clue that the cap has been opened since it was sealed. Packaging experts call this a tamper-evident feature.
“Tamper-evident” means that opening is easier to notice; it does not mean the package is impossible to open or that the contents are guaranteed safe. The ring cannot tell you when or by whom the bottle was opened. If a new bottle looks as if its cap has already been opened, do not drink from it; contact the store or maker.
Not every bottle leaves the ring behind
Caps are designed to fit particular bottle necks. On many single-use bottles, the ring separates from the cap and stays on the neck. Some returnable bottles use a different design: the ring can tear but remain attached to the cap. The details vary, but both designs make the first opening easier to spot.