You want to boil a two-minute egg. If you only have a three-minute timer (hourglass), a four-minute timer and a five-minute timer, how can you boil the egg for only two minutes?

If 21x = 79x, what is the value of x?

The host of a game show, offers the guest a choice of three doors. Behind one is a expensive car, but behind the other two are goats.
After you have chosen one door, he reveals one of the other two doors behind which is a goat (he wouldn't reveal a car).

Now he gives you the chance to switch to the other unrevealed door or stay at your initial choice. You will then get what is behind that door.

You cannot hear the goats from behind the doors, or in any way know which door has the prize.

Should you stay, or switch, or doesn't it matter?

Suppose we lay down two cups in front of you. One of the cups is filled with tea and the other one with coffee. Now we ask you to take a spoonful of tea and mix it with the coffee. At this moment, the coffee cup has a mixture of tea and coffee. You have to take that mixture (spoonful) and add it back to the tea.

Can you now tell if the cup of coffee has more tea or the cup of tea has more coffee?

What can go up a chimney down, but can’t go down a chimney up?

A seven-year-old kid challenged his classmates that he can make the number one disappear by adding something to it.

How can he do that?

Different lights make me strange by changing me into different sizes. What am I?

What shoe company does this rebus represent?

Can you find out the smallest number that can be conveyed as the sum of three squares in three unique ways?

I am a prime number.
The double of myself is equal to the square of me.

which number am I?