You're racing down the hall. An employee stops you and says, "We've got a problem." You assume you should get involved but can't make an on-the-spot decision. You say, "Let me think about it."
You've just allowed a "monkey" to leap from your subordinate's back to yours. You're now working for your subordinate. Take on enough monkeys, and you won't have time to handle your real job: fulfilling your own boss's mandates and helping peers generate business results.
