What goes up but never comes down?

10 people came into a hotel with 9 rooms and each guest wanted his own room. The bellboy solved this problem.
He asked the tenth guest to wait for a little with the first guest in room number 1. So in the first room, there were two people. The bellboy took the third guest to room number 2, the fourth to number 3, ..., and the ninth guest to room number 8. Then he returned to room number 1 and took the tenth guest to room number 9, still vacant.
How can everybody have his own room?

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

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

Who am I?

You are presented with three boxes. One of them has a red ball inside and the other two have a black ball inside each of them. You are asked to pick up the one with red ball and you pick one. Now, one of the other boxes is opened and it is found to have a black ball.

You are presented with a chance to change your box with the one that is left closed. Will you change your box? Why or why not?

I have branches, but no fruit, trunk or leaves. What am I?

What goes through cities and fields, but never moves?

The more it dries, the wetter it becomes. Can you tell what it is?

Astrology starts with the alphabet A, Biology starts with B, Psychology with P and Limnology with L.

Can you find an "ology" that begins with a P?

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?