The saucer has more surface area than the cup. Increasing the surface area, helps in evaporation and thus in cooling (Evaporation causes cooling effect). So, instead of drinking the hot cup of tea which may burn our tongue, we drink it in a saucer.
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Evaporation is a surface phenomenon . Saucer has a large surface area than cup. It will be easier for the particles of tea to be escaped from saucer rather than cup. Therefore we prefer to sip hot tea with a saucer rather than a cup .