By removing one matchstick and adding one matchstick, can you convert the word "ZOO" to an animal name?

Is the statement valid that it does not matter how much older a sibling is, eventually the younger one will be half as old as the older one?

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

A number with an interesting property:

When I divide it by 2, the remainder is 1.
When I divide it by 3, the remainder is 2.
When I divide it by 4, the remainder is 3.
When I divide it by 5, the remainder is 4.
When I divide it by 6, the remainder is 5.
When I divide it by 7, the remainder is 6.
When I divide it by 8, the remainder is 7.
When I divide it by 9, the remainder is 8.
When I divide it by 10, the remainder is 9.

It's not a small number, but it's not really big, either.
When I looked for a smaller number with this property I couldn't find one.

Can you find it?

The dirtier I get, the whiter I look. Who am I?

It has horns but can’t beep. It is like to bleat but It is not a sheep. What is it?

With the help clues provided in the image, can you find the 5 digit key?

What you cannot hold but it is yours?

A network of 20 x 10 squares is given to you.

Can you calculate how many unique squares and rectangles can be formed combining two or more individual squares ?

I have a hole in my back and legs I lack.
I live where I can’t breathe and I eat without teeth.
What am I?