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

How can you make number one disappear by adding something to it?

Here is a mathematical expression that also secretly tells a movie name. What movie is that?

Which is heavier: a ton of bricks or a ton of feathers?

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.

With the help clues provided in the image, can you find the 5 digit key?

A man is looking at a photograph of someone. His friend asks who it is. The man replies, “Brothers and sisters, I have none. But that man’s father is my father’s son.” Who was in the photograph?

A pond had 60 fish, out of which, all but 20 died.
How many fishes are now present in that pond?

A bridge is about to collapse. There are four people P, Q, R and S on one of the sides. Before the bridge collapses, they want to cross it. Now since the bridge is too weak, it can only stand the weight of two people at a time. Also, it is night time and nothing is visible. They have just one torch with them.

Now P takes one minute to cross the bridge, Q takes two minutes to cross, R takes five minutes to cross and S takes ten minutes to cross.

The bridge will collapse in seventeen minutes. How will they be able to cross the bridge before it collapses?

Take away the whole and some still remain. What is it?