what does ""balance peas on one's knife"mean?
A president, like all of us, can balance only so many peas on his knife at one time. There’s a time and place for adding to a president’s considerable challenges and there are days when delivering bad news is the last thing one should do. But it is those closest to the boss— any boss—who can best sense when those times are.

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This is saying that a president, or any person for that matter, can only handle so much at a time.

The outdated custom of eating peas with a butter knife took having an awareness (however slight) of the position, status, and possible movement of each pea in order to successfully eat them without dropping one, or all. In the same way, every person has a limit to what they are able to effectively manage, carry emotionally, or otherwise deal with, before their peas, so to speak, roll off of their knife.