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

I travel all around the world but always stay in the corner. What am I?

They are three errirs in this question. Can you find them?

I have 50 feet long of cloth, I need to create 50 handkerchiefs each size of 1 foot. I take one minute to cut a 1-foot handkerchief. So how long will it take to cut 50 handkerchiefs?

If all Goats are Sheep and all Sheep are Deers, are all Deers Goats ?

Eight of us go fourth, not back, to protect our king from an attack. What are we?

Can you find the key in the picture below?

There are six people in a family namely M, N, O, P, Q and R.

R and O are brother and sister.

N is the brother of Q's husband.
P is the father of M and grandfather of R.

How many male members are in the family?

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.

There is a box full of marbles,
all but two are blue,
all but two are green,
and all but two are red.

How many marbles are in the box?