The square root of number 121 is "11". What is the square root of the number "12345678987654321."?

The Puzzle: A girl, a boy, and a dog start walking down a road.

They start at the same time, from the same point, in the same direction.

The boy walks at 5 km/h, the girl at 6 km/h.

The dog runs from boy to girl and back again with a constant speed of 10 km/h. The dog does not slow down on the turn.
How far does the dog travel in 1 hour?

A man who was outside in the rain without an umbrella or hat didn’t get a single hair on his head wet. Why?

Why is a river so rich ?

What work can one never finish?

A crime was committed at baker street. Ibrahim Dakota who was shot in the stomach was the main suspect. Sherlock questioned the suspect. The conversation started as:
Sherlock: What's your story, Ibrahim?
Ibrahim: I was walking around baker street and suddenly a man from the back shot me. I ran as fast as I could to save my life".
Sherlock: That is enough (and ask the police to arrest him).

Why does Sherlock think he is the murderer?

What is the difference between a jeweller and a jailer?

What mathematical symbol can be put between 5 and 9, to get a number bigger than 5 and smaller than 9?

What number does the below three word/phrase identify and why?
1. Deck
2. Year
3. Singing in the Rain

Guess the hidden movie name?