Bernard,
Here is an exercise that I've done with business users, to illustrate the need for well-defined requirements in order to get the desired solution. It takes about 15 minutes, is interactive an fun, and works well if you have people team up in smaller break-out groups. (I've done with groups of 3 to as many as 8 working together on the exercise.)
I give them high-level, somewhat ambiguous requirements for something that is generally familiar to most people, then ask each group to draw their 'solution' that meets those requirements. Last time I did this exercise, I gave requirements for a type of boat - e.g., "Need a mode of transportation that will carry people on trips across water. Should be able to navigate smaller and larger bodies of water, and dock at designated pick-up and drop-off sites." I gave the teams 5-7 minutes to draw their solutions, then we posted them and everyone walked around to see the wide variety of solutions created. One team was very creative - they thought there might be a requirement to accommodate people who couldn't pay the full fare, so they included oars on their boat for passengers to work their way to the destination :). Then I asked people what assumptions they had to make for their solution, and finally gave them some more detailed requirements and asked how that would have changed the solution they came up with. E.g. "If there was a requirement for a maximum crew size of 3 people to run the ship, would that have changed your solution?" "If there was a requirement that the boat must carry at least 50 passengers, have separate crew quarters and a kitchen to serve meals, would that have changed your solution?" In most cases, the answers were almost unanimously, 'yes the solution would be different with detailed requirements'.
Another time, I used a robot - high-level requirements were something like "Need a motorized programmable household assistant to perform house and yard chores as requested by the household members". This one was fun as well, because I had even more variety in the detailed requirements, such as "Need the solution to navigate up and down stairs" and "Need the solution to be able to drive a car in order to pick up groceries or kids from school." Nearly everyone had put wheels on their robots, so none could handle stairs or drive cars.
It was definitely an eye-opener for everyone, and they all enjoyed the chance to get a bit creative as well. Let me know if you'd like more info, and I might be able to dig up the presentation I used...
Sandy