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?
If you were to put a coin into an empty bottle and then insert a cork into the neck, how could you remove the coin without taking out the cork or breaking the bottle?
What is the four-digit number in which the first digit is one-third the second, the third is the sum of the first two, and the last is three times the second?
You and your two friends are working in a multinational company. How can you three find out the average salary of you all without disclosing your own salary to the other two?