Pure programming puzzles
Despite our name, programming puzzlesprogramming puzzles aren't generally welcome here. I'm talking about challenges where you're given a task with some impossible-seeming restrictions and must use some clever trick or language feature to solve it.
Programming puzzles are closed most of ther time for not having an objective winning criterionobjective winning criterion: something that lets you compare submissions to decide a winner. A criterion for a valid solution is not enough -- others need to be able to do better and beat the solution. You could say the first valid solution wins, but some people find it unsatisfying that someone else can win before they even see the challenge. It's better when more people can participate in and enjoy a challenge.
If it makes sense for your challenge, you can say the shortest solution wins (code golf) or have some other metric. This is best for puzzles that can be solved by more than one method, so the search for better methods can be interesting. Beware that once someone posts a solution to the puzzle, others can see it and copy the key idea with a better-scoring implementation.