Why Aeroplane windows are round?

There is a place where if I stand up straight, I will still be considered sideways.

They came out at night, without being called.
They lost at day, without being stolen.
Who are they?

A detective visits his friend John just to discover that John's neighbour Dr Jacob was murdered. Dr Jacob, who is a mathematician wrote the murderer's name in a code word "Hcqtfz". There were five suspects
1. Ajit
2. Pankaj
3. Ganpat
4. Deenu
5. Hariya

The Detective was instantly able to find the murderer.
Can you?

What goes up but never comes down?

You board a train. The train will have to enter a tunnel soon. You are claustrophobic. Which place is the best for you to sit?

See the given image carefully. What you have to do is move the blue checkers in the position of the black checkers and vice versa. You are only allowed to move the checker to an adjacent empty space. Do it in the least possible moves.

In a boat, the father of a sailor's son is sitting with the son of the sailor. However, the sailor is not present on the boat.

Can this even be possible?

In front of you, there are 9 coins. They all look absolutely identical, but one of the coins is fake. However, you know that the fake coin is lighter than the rest, and in front of you is a balance scale. What is the least number of weightings you can use to find the counterfeit coin?

You may enter, but you may not come in. I have space, but no room. I have keys, but open no lock. What am I?