A brain teaser is a form of puzzle that requires thought to solve. It often requires thinking in unconventional ways with given constraints in mind; sometimes it also involves lateral thinking. Logic puzzles and riddles are specific types of brain teasers. One of the earliest known brain teaser enthusiasts was the Greek mathematician Archimedes. He devised mathematical problems for his contemporaries to solve. One can argue about the answers of many brain teasers; in the given example with hens, one might claim that all the eggs in the question were laid in the first day, so the answer would be three. Some brainteasers can have many ways to get the solution.
If you are looking for a challenge you are in the right place. Brainteasers sometimes force students to think about situations a little differently so here you can find 74 worksheets including quizzes, puzzles, and riddles. This brainteaser worksheet has a simple riddle that is not really related to ESL but that your students might enjoy if you structure it as a short reading and speaking activity where students have to read the riddle and also present their answers to the class. The answer is posted as the first comment and is really quite clever. If you think that perhaps another type of activity would be better for your class, there are many more to choose from. If the preview does not give you a clear enough idea of the worksheet, simply download it onto your computer and look over the whole thing.
Brainteasers can be fun for students and are definitely challenging but unless there is a direct connection to ESL, try to keep these activities short so that students can maximize their time studying more common, everyday topics. On the other hand, some would argue that brainteasers help students with their problem solving and logic skills which are both very important for learners of any age. To help you decide whether or not brainteasers are right for you class, try one out and see how your students react. If they are enthusiastic about solving puzzles then you should feel free to include them in some of your lessons but if students struggle, give up, or become frustrated consider other activities instead.