The Twin Earth thought experiment is a famous philosophical exercise introduced by Hilary Putnam in 1973 and 1975 to challenge traditional ideas about language and the mind. [1, 2]

The Core Setup

The Duplicate Planet: Imagine a planet called Twin Earth that is an exact, molecule-for-molecule duplicate of Earth, sharing the same history, geography, and culture. [1, 2, 3]
The Doppelgängers: You have an exact duplicate (a twin) on Twin Earth. Every thought, feeling, and neural state in your brain is identical to your twin's. [1, 2, 3]
The One Difference: On Twin Earth, the clear liquid that fills the rivers, lakes, and oceans—and comes out of the faucets—is not \(\text{H}_2\text{O}\). It is a completely different chemical compound known as \(\text{XYZ}\). [1, 2, 3]
The Illusion: \(\text{XYZ}\) looks, tastes, and feels identical to \(\text{H}_2\text{O}\). If you visit Twin Earth (or your twin visits Earth), neither of you would initially notice any difference. [1, 2, 3]
The Timeline: The experiment is often set in a period before modern chemistry (such as the year 1750), when people on both planets have no idea what the underlying molecular structure of their respective liquids is. [1]

The Scenario

You and your twin both look down at a glass of clear liquid and say the exact same sentence:

"Water is wet." [1]

Despite having identical brain states, identical mental images, and identical thoughts: [1]

When you say "water," your word refers to \(\text{H}_2\text{O}\).
When your twin says "water," their word refers to \(\text{XYZ}\). [1]

The Conclusion

Putnam used this scenario to argue for semantic externalism—summarized by his famous slogan: [1, 2]

“Meanings just ain't in the head.” [1, 2]

The experiment aims to prove two main things:

Mental states do not determine linguistic reference: Two people can be in the exact same internal psychological state and yet use the same word to mean two different things. [1, 2]
Environment and history matter: The meaning of a natural-kind term (like "water," "gold," or "tiger") depends partly on the actual physical reality of the external world and your causal interaction with it, not just your internal thoughts. [1, 2, 3]

Read more about this theory on the Stanford Encyclopedia of Philosophy or review the Wikipedia Overview. [1, 2]