Six Glasses

Six identical glasses are placed in a row on the table – first three filled with water, and then three empty ones. Can you move just one glass, so that empty and full glasses alternate?

Take the second full glass, pour all the water into the second empty glass, and then put it back in its place.

Buried Up to Neck

Three friends, Adam, Bob, and Charlie are buried in the sand up to their necks, all facing West. Charlie can see both Adam and Bom, Bom can see only Adam, and Adam cannot see anyone. Black and white hats are placed on their heads. The three friends are told that there is at least one hat from each color, and then they are asked whether anyone can guess the color of their own hat.

After a few minutes, one of them answers. Who is that?

If Adam and Bob had hats with identical colors, then Charlie would immediately be able to deduce that his hat has the opposite color. Charlie doesn’t do that, so Adam and Bob are able to figure out that their hats have opposite colors. Since Bob is the one who can see the color of Adam’s hat, he is the one that answers the question.

Ancient Coins

Suppose I show you two ancient coins. The first one is dated 51 B.C., the second one is marked George I. Which one is counterfeit?

Both of them. People who lived 51 years before Christ didn’t know about Jesus yet. When King George I was ruling, he was the first king with this name, and so he was just called George.

Third Business Day

What is the chance that the third business day of a month is Wednesday?

Assuming there is an equal chance that the given month begins with any of the seven days of the week, the answer is 3/7. That’s because Saturday and Sunday are non-working days, and therefore if the month starts with any of them, the third business day will still be Wednesday.

We note that 3/7 is actually an approximation of the actual answer, since the frequencies with which the days of the week appear as first in a given month, are close to but not equal to 1/7 each.

Ping Pong Ball

Your last ping pong ball falls down into a narrow pipe embedded in concrete one foot deep. How can you get it out undamaged if all you have is your tennis paddle, your shoelaces, keys, wallet, and a plastic water bottle, which does not fit into the pipe?

Using the plastic bottle, pour water into the pipe so that the ball will rise up.