Always in you, Sometimes on you;

If I surround you, I can kill you.

What am I?

Can you place three balls such that the equation shown in the picture holds true?

Take away my first letter, then take away my second letter. Then take away the rest of my letters, yet I remain the same. What am I?

There are people and strange monkeys on this island, and you can not tell who is who (Edit: until you understand what they said - see below). They speak either only the truth or only lies.
Who are the following two guys?
A: B is a lying monkey. I am human.
B: A is telling the truth.

Can you name three consecutive days without using the words Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday?

If we change the South-East direction into North and North-East into West and all others similarly.
Can you find out which direction will be in the place of South-West direction?

I left my campsite and hiked south for 3 miles. Then I turned east and hiked for 3 miles. I then turned north and hiked for 3 miles, at which time I came upon a bear inside my tent eating my food! What colour was the bear?

A father's child, a mother's child, yet no one's son.

Who am I?

You see me once in June, twice in November, and not at all in May. What am I?

Why do we preferably have round manhole covers and not square ones?