When I get multiplied by any number, the sum of the figures in the product is always me. What am I?

Why 1729 is a magic number? Answer Logically.

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.

Always in you, Sometimes on you;

If I surround you, I can kill you.

What am I?

A man started to town with a fox, a goose, and a sack of corn. He came to a stream which he had to cross in a tiny boat. He could only take one across at a time. He could not leave the fox alone with the goose or the goose alone with the corn. How did he get them all safely over the stream?

What has many keys but can’t open a single lock?

I am weightless, but you can see me. Put me in a bucket, and I'll make it lighter. What am I?

I make two people out of one. What am I?

I am soft and cuddly that soothe your heart. But, If you pick my last name I am going to tear you all apart. Who am I?

Twice ten are six of us,

Six are but three of us,

Nine are but four of us;

What can we possibly be?

Would you know more of us?

Twelve are but six of us,

Five are but four, do you see?

What are we?