If You Can't Stand the Heat, Get Out of the Kitchen
If there's a task you cannot manage properly due to the pressure involved, then you should discontinue from doing it.
With the ovens turned on and the stoves set to bake, the kitchen rapidly changes from a comfortable place to prepare food, into a heated spot of sweaty work. The rising temperatures create a warm enviroment that is tolerable to some, but insufferable to others. For those that grow uncomfortable because of the heat, well, it'd probably be best for them to leave the kitchen if they can't cope, and leave the work to someone who can.

Before becoming president, it's thought that Harry S. Truman coined the phrase at least as early as 1942. In a Tennessee newspaper called Kingsport Times, from 1942, it reads:

"Favorite rejoinder of Senator Harry S. Truman when a member of his war contracts investigating committee objects to his strenuous pace: 'If you don't like the heat, get out of the kitchen."

The Facts on File Encyclopedia of Word and Phrase Origins, by Robert Hendrickson, mentions there may possibly be some evidence that Truman got it from his military adviser, General Harry Vaughn.
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* After landing a job as a surgeon, Bill later regretted it when he got queasy at the sight of blood; thus, the lead surgeon told him that if he can't stand the heat, then he needs to get out of the kitchen and find a more suitable job.