''What or When ''

An ExtremeRule: "You can tell us what, or when."

Context: Your team size is set. Everyone knows about what level of quality is required.

Customers/management can tell the team ''What'' 
is needed, and the team can say ''When'' they'll be done.

Or customers/management can tell the team ''When'' 
it is needed, and the team can say ''What'' they can get done by then.

Some people think you can dictate resources, quality, scope and time. Some people think pigs will fly. Many of these are the same people.

See also FourVariables.
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Here is something similar.  You walk into a candy store with $1.  You can say "I want 2 lollipops and a sourball" and the shopkeeper will tell you how much that costs, or you can say "I have $1.  Please give me all the lollipops and sourballs I can have for that."  But, you can't walk in and say "Here is $1.  I want 15 lollipops and 50 sourballs.  

The reason is simple.  There is a real cost associated with everything.  People who don't believe this are very hard to work with.