The square root of number 121 is "11". What is the square root of the number "12345678987654321."?

When you say goodbye, I am what you need to tie.
I come as a couple and go wherever you run.
Who am I?

What can you deduce from the following BrainBat?

CRY
LMKI

At a party, there are five people and a whole round cake lying at the centre of the table. Only four people will make a cut and take their piece and the last one will get the remaining piece on the table. How can they make sure that everyone gets a 1/5th of the piece?

A man stands on one side of a river, his dog on the other. The man calls his dog, who immediately crosses the river without getting wet and without using a bridge or a boat. How did the dog do it?

I look flat, but I am deep. Hidden realms I shelter. Lives I take, but the food I offer. At times I am beautiful. I can be calm, angry, and turbulent. I have no heart but offer pleasure as well as death. No man can own me, yet I encompass what all men must have. What am I?

Nine marbles are arranged in the image below. Can you slide two marbles to form a square?

In a basket of apples,
when counted in twos, there was one extra
when counted in threes, there were two extra when counted in fours, there were three extra
when counted in fives, there were four extra
when counted in sixes, there was five extra.

However, if the apples were counted in sevens, no extra apple was left. Can you calculate the minimum number of apples that were present in the basket?

Prove that twice of ten is the same as twice of eleven.

Get total of 120 by using five zeros 0,0,0,0,0 and any one mathematical operator.
Do you ?