Four glasses puzzle facts for kids

The four glasses puzzle is a fun brain teaser! It's also known as the blind bartender's problem. This puzzle became famous when Martin Gardner wrote about it in Scientific American magazine in 1979. It challenges you to think logically to solve a tricky situation.

What is the Four Glasses Puzzle?

Imagine four drinking glasses placed on the corners of a square tray called a Lazy Susan. Some glasses are standing upright, and some are upside-down.

A person is blindfolded and sits next to the tray. Their job is to make all the glasses face the same way. This means all of them must be upright, or all of them must be upside-down. When they succeed, a bell rings!

Here are the rules for the blindfolded person:

• In each turn, they can feel any two glasses.
• After feeling them, they can choose to flip one, both, or neither of those two glasses.
• After each turn, the Lazy Susan tray spins around randomly. This means the person won't know which glasses are where anymore.
• The goal is to find a plan that always works, no matter how the glasses start. It cannot depend on luck!

How to Solve the Puzzle

There's a clever plan that guarantees you'll solve the puzzle in five turns or less. Here's how it works:

• Turn 1: Pick two glasses that are opposite each other (like on a diagonal). Turn both of them so they are upright.
• Turn 2: Choose two glasses that are next to each other. Because of the first step, at least one of these two glasses will be upright. If the other glass is upside-down, turn it upright too. If the bell doesn't ring, it means you now have three glasses upright and one upside-down.
• Turn 3: Pick two glasses that are opposite each other again. If one of them is upside-down, turn it upright, and the bell will ring! If both are already upright, turn one of them upside-down. Now, you will have two glasses upside-down, and they will be next to each other.
• Turn 4: Choose two glasses that are next to each other. Flip both of them over. If they were both facing the same way (both up or both down), the bell will ring. If not, you now have two glasses upside-down, and they must be opposite each other.
• Turn 5: Pick two glasses that are opposite each other. Flip both of them over. The bell will ring! You have solved the puzzle.

Puzzle Variations

This puzzle can be changed in different ways. For example:

• If you only have two glasses, you can solve it in just one turn! You just flip one of them.
• With three glasses, there's a plan that works in two turns.
• However, for five or more glasses, there isn't a simple plan that always works in a set number of turns.

Sometimes, the puzzle is changed to allow you to check more than two glasses at a time. If you can check enough glasses in each turn, you can still find a way to solve it!

Learn more about the Four Glasses Puzzle