Puzzles and Algorithms

This time, I would like to talk about a clever way of general algorithmic puzzle solving technique.

My first example is the celebrity problem: A celebrity is someone who knows nobody but is known by everybody. In a group of n people with 1 celebrity among them, what is the minimum number of questions that you should ask to find the celebrity for sure. The questions are of the form: "Do you know THAT guy?"

The second example is the managers-engineers puzzle: There are
n people in the building, each either an engineer or a manager. The problem facing the FBI is to separate engineers from managers. Each of the n people knows the status of all the others. The questions are of the form: "Is this person an engineer or a manager?" The engineers always tell the truth. What makes it hard is that the managers may not tell the truth. In fact, the managers are evil geniuses who are conspiring to confuse the interrogators. Under the assumption that more than half of the people are engineers, can you suggest a strategy for the FBI to find one engineer with at most n-1 questions?

Answers are to be posted... The way will be to eliminate as much people as possible with a single question while still maintaining the problem invariant.
Hint: Think it backwards ! Spotting an engineer or a celebrity may be hard. But it may be easier to spot a manager or a noncelebrity (respectively). Then we will have a subproblem...