What is common among 11, 69, and 88?

If I remove one from eleven it becomes Ten and if I remove one from nine also becomes Ten. How?

Move four matchsticks to the below square to form three squares?

What has one letter and starts with an E?

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?

You are an expert on paranormal activity and have been hired to locate a spirit haunting an old resort hotel. Strong signs indicate that the spirit lies behind one of four doors. The inscriptions on each door read as follows:

Door A: It's behind B or C
Door B: Its behind A or D
Door C: It's in here
Door D: It's not in here

Your psychic powers have told you three of the inscriptions are false, and one is true. Behind which door will you find the spirit?

I make a loud sound when I’m changing. When I do change, I get bigger but weigh less. What am I?

If a zookeeper had 100 pairs of animals in her zoo, and two pairs of babies are born for each one of the original animals, then (sadly) 23 animals don’t survive, how many animals do you have left in total?

How many letters are in the alphabet?

Irene killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?