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

John is the son of Jacob.
So,
Jacob is the_____of John's father?

Quickly Solve this riddle

7 - 4 + 3 * 0 + 1 = ?

Why is the Hole below a Lock?

Give it food and it will live; give it water and it will die.

I want to fill my bucket using both cold and hot water.
I have two taps for both cold and hot water. The hot water tap fills the bucket in exact 6 hours and the cold water tap fills the bucket in exact 4 hours.
I turn both of them simultaneously but I forgot to turn off another tap which removes the water out of the bucket. This tap can empty the bucket in 12 hours.

How Long will it take to fill the bucket?

You are in a room with no metal objects except for two iron rods. Only one of them is a magnet.
How can you identify which one is a magnet?

I had an infinite supply of water and 5 litres and 3 litres jars.

How would you measure exactly 4 litres in the least number of steps?

what does the below rebus means?

Decode below rebus
D
N
A
T
S , D