The explanation is partly the frog-in-hot-water phenomenon (he'll jump out if you drop him in boiling water, but not if you put him in cold water and slowly heat it to boiling).
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The idea of slowly heating the water instead of dropping a frog straight into boiling water is what's meant by the phrase "frog in water".
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The real explanation has nothing to do with the frog-in-boiling-water phenomenon.