What occurs twice in a week, once in a year but never in a day?

A man has a barrel filled with oil that weighs 100 pounds, and then he puts something into it. Now the barrel weighs less than 100 pounds. What did he put in the barrel?

A mother bought three dress for her triplets daughters(one for each) and put the dresses in the dark. One by one the girls come and pick a dress.
What is the probability that no girl will choose her own dress?

The following alphametic has only one solution. Can you find it?
ENLIST + SILENT + LISTEN = ANAGRAM

PS: You may start the number from 0.

2, 3, 5, 9, 17, _ What is the next number in the sequence?

One absent-minded ancient philosopher forgot to wind up his only clock in the house. He had no radio, TV, telephone, internet, or any other means for telling time. So he travelled on foot to his friend's place a few miles down the straight desert road. He stayed at his friend's house for the night and when he came back home, he knew how to set his clock. How did he know?

The centre of the third figure is empty. What digit should fit inside?

Find the next number in the series

12, 20, 40, 72, 116, 172 ?

What can go through glass without breaking it?

This is a classy optical illusion puzzle. What more you can find except bricks in the image below?