A plastic trash bag lines a kitchen bin, containing food scraps, cardboard, and cans. Normally, even when brimming, it holds firm with no breaks or leaks.
Gripping the yellow drawstring handles, you hoist it straight up from the bin. Up to the boundary, the entire load rises as one piece, waste staying put inside.
The threshold crosses at the instant the bottom seam's plastic ruptures under the total downward pull.
Before that split, the bag supports and lifts cleanly. The split creates an immediate gash; contents cascade out through it onto the floor.
The bag, once enclosing the waste, now lies open with its hold broken. This marks the line from contained load to spreading spill.