What does this rebus mean?

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Many have heard it, but nobody has ever seen it, and it will not speak back until spoken to. What is it?

I am weightless, but you can see me. Put me in a bucket, and I'll make it lighter. What am I?

Use the numbers 2, 3, 4 and 5 and the symbols + and = to make a true equation. Conditions: Each must be used exactly once and no other numbers or symbols can be used.

Solve the following series:
AZ, GT, MN, __, YB

A chess tournament is taking place on knock-out terms (the one who loses the match is out of the game).
(a) If 10 matches are played in total, how many players participated?
(b) If 20 players took part in the tournament, how many matches were played?

Forwards I am heavy, backward I am not. What am I?

John’s parents have three sons: Snap, Crackle, and what’s the name of the third son?

The dirtier I get, the whiter I look. Who am I?

John and Jacob are twin brothers. One of them is a liar whereas the other one is a truth-teller.

I called their home number and one of them picked up.
I asked. 'Does John lie?' The person replied with a yes.

Whom did I talk with, John or Jacob?