Find the missing value of "?" in the letter sequence below.

CAD
EBG
KBM
OCR
DDH
NL?

If an electric train is travelling south, then which way is the smoke going?

It can be easily calculated that the digits 0 to 9 can be arranged into 3628800 distinct ten-digit numbers.

But do you know how many of these numbers are prime?

I live in a bowl. I can swim. I have a tail. I also have fins and big eyes. What am I?

What demands an answer, but asks no questions

This is a most unusual paragraph. How quickly can you find out what is so unusual about it? It looks so ordinary, you'd think nothing was wrong with it. Actually, nothing IS wrong with it. But it is not as ordinary as you might think. If you think about it for a bit, you will find out why it is truly so unusual. So what is it? What is so unordinary about this paragraph?

I am a white box having no key or lid.
Yet there is a golden treasure inside it.

What Am I?

I am white when I am dirty, and black when I am clean. What am I?

Twenty frogs are sitting on a log floating on the surface of a river. Two of them decide to jump off into the water.

How many frogs are there on the log at this moment?

Before Mount Everest was discovered, what was the highest mountain in the world?