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It is said that a liquid boils when its vapor pressure equals to the external pressure of its environment (e.g. atmospheric pressure). That’s because the bubbles of water vapor that form will be crushed if the vapor pressure is below the atmospheric pressure. However, it’s the fluid’s pressure that actually does the crushing, isn’t it? And the fluid pressure varies with depth, with only the fluid at the top of the liquid being equal to atmospheric pressure. Shouldn’t we say that the liquid boils when its vapor pressure is equal to the greatest fluid pressure in the liquid?