Assertion: flask's opening | /r/HasProperty | necessary to prevent pressure
Facets:
value: in this case
type: TEMPORAL
Source sentences:
text: Larger vacuum flasks store liquids that become gaseous at well below ambient temperature, such as oxygen and nitrogen; in this case the leakage of heat into the extremely cold interior of the bottle results in a slow boiling-off of the liquid so that a narrow unstoppered opening, or a stoppered opening protected by a pressure relief valve, is necessary to prevent pressure from building up and eventually shattering the flask.