Grades 3–5 · Everyday Life
How does a bottle cap keep a drink from leaking?
A screw cap does two linked jobs. Its threads hold it on the bottle, while a sealing surface closes the small escape path around the opening. Threads alone usually do not make the whole seal.
In the diagramThe thread holds; the sealing surface closescap presses downflexible linerthreadSealing contact must continue all around the rimCutaway: the pictured design uses a separate liner.
As the cap turns, its thread moves it along the matching thread on the neck. In the illustrated design, this brings a flexible liner down against the bottle’s rim. The liner can bend slightly and press into small uneven places.
For a liquid to leak out, it needs a connected gap from inside to outside. Contact all around the rim interrupts that route. The cap, rim and liner must fit together; a crack or trapped piece of material can leave a path.
Many plastic caps instead use a shaped lip that seals inside or against the neck. The details differ, but the important connection is the same: one structure holds the closure in position so another surface can maintain the seal.
About this explanation
Starting knowledge: Recognize a bottle opening and a screw thread; follow contact around an edge.
Sources
- BERICAP — Filling Technology: closure sealing systems