Here's a riddle I just came up with. There's no trickery involved, and the solution is simple.
100 people are applying for 100 jobs at a company, and the company wants to test their intelligence. If they pass the test, they all get jobs. If they fail, none of them get jobs. The manager brings them into a room for a meeting and explains their task. In the building, there's a closet with nothing in it except for one three-way lamp (a lamp with a bulb that can give three different levels of brightness). They're all allowed to examine it ahead of time. After the meeting, they'll be allowed to confer briefly, then they'll all be isolated and unable to communicate with each other. Each applicant will be given a random number between 1 and 100 (and numbers can repeat themselves). In random order, they'll be led one at a time into the room and allowed to turn the lamp on and off, or change the brightness level, as they wish. There is no other communication possible with the other applicants once the numbers are given out. The last person into the room must tell the manager whether the sum of all of the numbers the applicants were given is divisible by four.
What is their strategy?