What is full of holes but still holds water?

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

Find three numbers such that

* When we multiply three numbers, we will get the prime numbers.
* The difference between the second and the first number is equal to the third and second.

You see a boat filled with people, yet there isn’t a single person on board. How is that possible?

A man lives on the fifteenth floor of an apartment building. Every morning he takes the elevator down to the lobby and leaves the building. In the evening, he gets into the elevator, and, if there is someone else in the elevator, or if it was raining that day, he goes back to his floor directly. Otherwise, he goes to the tenth floor and walks up five flights of stairs to his apartment. Can you explain why he does this?

What has a bottom at the top?

Prove that the below maths equation is true.

8 + 8 = 91

Remove two matchsticks to make the below equation correct.

Can you replace the X (both small and big x) with the numbers between 1 to 19 such that all the rows of 3 numbers between X (big x) sum to 23?

Baseball bat and ball cost $100. If the bat cost $99 more than the ball, what is the cost of each?