~~~vacuum flask~~~

Silvered walls are poor radiators and good reflectors thus reflecting heat back into the flask, reducing heat loss by radiation. The plastic stopped is a poor conductor of heat thus reducing heat loss by conduction. The stopper also prevents warm air from escaping through convection currents. The vacuum prevents heat loss by conductionn and convection as both require medium for heat transfer. Glass is a poor conductor of heat.

~~~Fish can survive underwater even when the top of the pond is frozen over~~~

During winter, the surface of the pond gets cooled first. It contracts and becomes more dense, thus sinking to the bottom of the pond. Warmer and less dense water from the bottom rises to take its place. As the surface of the pond is cooled further, it longer contracts but expands and becomes less dense. Water at the bottom of the pond remains at 4 degree celsius, thus fish can still survive.