A jar sits on the kitchen counter, its lid clamped firmly to the glass. The seal holds tight, keeping contents isolated from the outside air. This is the stable state: complete enclosure, no movement or sound when turned.
Heat reaches the jar. Inside, pressure rises against the unyielding lid.
The limit approaches. The lid stays fixed until internal force overcomes the grip.
The threshold crosses at the sharp pop. The lid lifts slightly as air breaks through.
Before: solid, airtight hold. After: lid spins free, contents exposed. The jar shifts from sealed to open.
This marks the line where enclosure ends.