Hii:) what’s the use of a cork in vacuum flask?

In terms of Physics in the study of transfer of thermal energy, the cork is used to reduce heat loss by convection. If hot water is poured into the vacuum flask, the air nearer the hot water would gain heat, expand, become less dense and rise up. The denser, cooler air from the surrounding sinks to the vacuum flask. This would result in the hot water losing heat over time. Thus, a cork is used to reduce heat loss by preventing the set up of convection currents.