Explain the following observations

(a) Birds usually fluff up their feathers during cold weather.
By fluffing up their feathers, birds trap tiny air pockets within their feathers. Since air is a poor conductor of heat, the bird loses less heat during cold weather.

(b) Cooking utensils, kettles and boilers are usually made of aluminium or stainless steel.
Aluminium and stainless steel (metals) conducts heat well. This helps to conduct heat quickly to the food to enable it to be cooked faster.

(c) Sawdust is used to cover ice blocks.
Sawdust is a bad conductor heat. By using sawdust to cover ice blocks, heat transfer from surroundings to ice is reduced, hence, melting of ice blocks is delayed.

(d) A black car becomes hotter than a white car when both are parked under direct sunlight for several hours.
Black surfaces are better than white surfaces in absorbing radiation. Thus, the black car absorbs more heat from radiation as compared to the white car, resulting in a hotter temperature for the black car.