What does below rebus means ?

Why do you find that most of the buildings are deprived of the 13th floor?

What has a bottom at the top?

There was once a college that offered a class on probability applied to the real world. The class was relatively easy, but there was a catch. There were no homework assignments or tests, but there was a final exam that would have only one question on it. When everyone received the test paper it was a blank sheet of paper with a solitary question on it: 'What is the risk?'.Most students were able to pass, but only one student received 100% for the class! Even stranger was that he only wrote down one word!
What did he write?

What has hands, but can’t clap?

A bomb exploded in a room full of married couples and no one died. How?

Find the smallest sentence with all the English alphabet.

What kind of tree can you carry in your hand?

There are forty elephants and they have forty-fore heads. How can this be possible?

What is the last thing you take off before bed?