I have no doors but I have keys, I have no rooms but I have space, you can enter but you can’t leave! What am I?

What goes up but never comes down?

Christina has four daughters, and each of her daughters has a brother. How many children does Christina have?

Can you circle exactly four of these numbers such that the total is twelve?

1 6 1
6 1 6
1 6 1
6 1 6

What has six faces, but does not wear makeup, has twenty-one eyes, but cannot see?

Imagine a box with two cogwheels, one big with 24 teeth and one small with 8 teeth. The big one is firmly attached to the center of the box which means it does not turn or move while the small one rotates around the big one.

How many times do you think that the smaller wheel will turn compared to the box when it turns once around the big one?

Rectify the following equality 101 - 102 = 1 by moving just one digit.

Solve below popular number sequence
314 159 265 358 979 323 846 ?

Here is what you have to do. You have to throw a ball as hard as you can but it must return back to you even if it does not bounce at anything. Also, you have nothing attached to the ball. There is no one on the other end to catch that ball and throw it back at you.

How will you do it?

A chess tournament is taking place on knock-out terms (the one who loses the match is out of the game).
(a) If 10 matches are played in total, how many players participated?
(b) If 20 players took part in the tournament, how many matches were played?