Can you make four (4) nines (9) equal 100?

There are 2 people near the river and both of them want to get on the other side. The boat can only take one of them. But they got across.
How?

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.

Akbar called Birbal and asked him to draw a line on the floor. He asked Birbal to make that line smaller without erasing it. Birbal smiled and did it before Akbar could blink his eyes.

How did he managed to do it?

A seven-year-old kid challenged his classmates that he can make the number one disappear by adding something to it.

How can he do that?

Three doctors said that Bill was their brother. Bill says he has no brothers. How many brothers does Bill actually have?

My thunder comes before the lightning; my lightning comes before clouds; my rain dries all the land it touches. What am I?

What is black when you buy it, red when you use it, and gray when you throw it away?

A new Engineering building containing 100 offices had just been completed. John was hired to paint the numbers 1 to 100 on the doors. How many times will John have to paint the number nine (9)?

What does below science rebus mean?

Ag Ag Ag
Ag SPOON Ag
Ag Ag Ag