Every loose material has a property known as the angle of repose. It is the angle that is naturally formed when a material is loosely spilled out, for example to be stored:
Give the development team an amount of loose substance such as salt of sugar and ask them to produce a measurement of the angle of repose of that substance. Then leave the room. Give them a protractor and leave the room.
Conclusions after the task is executed:
When you are back in the room, the engineers will have hopefully measured the angle in the experiment of spilling out the substance and measuring the angle more or less accurately. Explain to them that although a theoretical computation of the angle is possible, it would be extremely complex, compared to simply doing the experiment they have just done.
Then, this metaphore could be extended to cover topics like:
- empiricism as the method for solving (complex) problems (even if this particular is just complicated)
- TDD - experimenting with the production code to see when it passes a test, rather than trying to mentally execute it
- developing a story based on unclear or incomplete knowledge of scope, in order to get feedback, rather than struggling to understand and "lock" all of the story first