A Scientist was fifty years old in the year 2000 but he was forty years old in the year 2010. How can this be possible?

PS: It has a logical answer.

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

You can hear me, but you can't touch or see me.

What am I?

If Bihan is 10, Anirudha is 20, and Kim and Seal are both 5, but Rikold is 10, how much is Kenniger by the same trend?

What does the below rebus riddle means?

What is the difference between a jeweller and a jailer?

Using Only Five 5's and any mathematical operator make sum as 37

If,

20 % 5 = 24

21 % 7 = 33

42 % 6 = 67

Then;

66 % 6 = ?

Which wheel of car does not rotate when you turn your car right?

I am a type of vehicle that has two wheels and is powered by humans. What am I?
Hint 1: You can ride me for exercise.
Hint 2: I am environment-friendly.