OK here's a good, classic one. Now, I belive about 50% of the regular posters might be familiar with it, so I ask for you guys to not reply with the answer if you already know it. It's actually kinda easy:
Imagine you have with you a wolf, a rabbit and a carrot, and you are walking in the woods and you see a river. You must cross this river by using a small boat that rests on the edge of the river, but it's so small you can only bring one thing/animal with you. If you leave the rabbit alone with the carrot, the rabbit eats the carrot. If you leave the wolf with the rabbit, the wolf eats the rabbit. The wolf dosen't eat the carrot. Now, being able to carry one animal (or the carrot) at once, how can you cross the river?