Why Aeroplane windows are round?

No matter how little or how much you use me, you change me every month. What am I?

I am the fastest animal but cannot climb the tree. What am I?

Look at this sequence from top to bottom. What is the next number in the sequence?
1
11
21
1211
111221
312211

On a certain day, John celebrated his birthday. Two days later, his older twin brother Jacob celebrated his birthday.

Is this even possible? How can it be?

I’m tall when I’m young, and I’m short when I’m old. What am I?

That attorney is my brother, testified the accountant.

But the attorney testified he didn't have a brother.

Who is lying?

Replace the question mark in the picture below.

Below is the sum of symbols in each row and column in the figure. Analyzing the figure, can you find out the value of the symbols?

During an interview, the interviewer ordered hot coffee for the candidate to relieve the stress. The coffee was kept before him. After a minute, the interviewer asked him, 'What is before you?' He replied 'Tea'.

The candidate was selected immediately. Why?