Quantum decoherence

Quantum decoherence is like a game of telephone between people. When people are playing a game of telephone, they each whisper a secret message to the next person in the circle. The secret message gets a little bit different every time it's passed on until the last person in the bunch says something completely different than the first person said.

Quantum decoherence is the same thing, but with particles such as atoms and molecules. When a particle's quantum state changes, it passes that change on to the other particles it comes in contact with. As each particle passes the change on to the next, the original quantum state gets a little bit different until it becomes unrecognizable. This is why we can't measure and observe quantum states, because they keep changing every time we look at them!