Why should we not throw
(a) used tea leaves into sink?
(b) cooking oil and fats down the drain?

(a) Used tea leaves should not be thrown in kitchen sinks as they clog drains and prevent the free flow of oxygen that interferes with the decomposition process. They should be thrown in dustbins.

(b) Cooking oil and fats should not be disposed off down the drains as these get hardened and block pipes. They reduce the flow of water and result in stagnant water, which provides a breeding ground for mosquitoes and flies and spreads infectious diseases.