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: In a [[Closed System|closed system]] [[mass]] cannot enter or leave.
: In these [[Closed System|closed system]]s the law of [[Conservation of Mass]] can be [[observed]] but the law of [[Conservation of Energy]] cannot.
: A [[Closed System|closed system]] can be made by sealing a container with a lid. For example a [[Test Tube|test tube]] with a [[bung]] or a [[Conical Flask]] with a balloon to collect [[gas]]es that would otherwise escape.