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Dear Yahoo!:
Does blowing on hot soup really cool it down?
Gary
Asbury, New Jersey

Dear Gary:
Your question brings to mind Aesop's fable about a cold, hungry man who was lost in the woods. A kind satyr took pity on him and invited him into his home. At one point, the man blew on his hands to warm them, and later, when the satyr served him dinner, the man blew on his soup to cool it.

The satyr kicked the man out, stating "I will have nought to do with a man who can blow hot and cold with the same breath." (Hence the expression about an unpredictable person "blowing hot and cold.") But we digress, so let's get to your question. Did the man give up his nice warm accommodations for "nought," or did he know what he was doing when he blew on his soup to cool it?

It appears blowing on your soup does have a valid scientific effect. According to this light-hearted transcript from a scientific radio show broadcast by Indiana University, it's caused by evaporation.

When you sit down to that steaming bowl of soup, the faster-moving, hotter particles "leap off the surface," evaporating and leaving the slower-moving, cooler particles behind. But these evaporated particles form a little cloud of vapor above your bowl, saturating the air and preventing any more evaporation. When you blow on your soup, you disperse the vapor cloud. This clears the air, so to speak, for more hot particles to evaporate, thus cooling the soup.

Of course, just 'cause it works doesn't mean it's considered proper or polite.

http://ask.yahoo.com/20061220.html