Here are the rules:
* I'm going to tell you about things I like and things I don't like.
* These have nothing to do with my actual personal preferences. My likes and dislikes are determined by a rule that you must discover.
* When you think you have figured out the rule, don't tell me (or the class) the rule. Instead, give me a sentence of something I would like and dislike. I will tell you if I agree or disagree.
I like jelly, but I don't like jam.
I like yellow, but I don't like blue.
I like dresses, but I don't like skirts.
I like food, but I don't like eating.
I like pillows, but I don't like cushions.
I like carrots, but I don't like squash.
I like football, but I don't like sports.
I like summer, but I don't like spring.
I like yelling, but I don't like screaming.
I like butter, but I don't like margarine.
I like cookies, but I don't like cake.
I like hoodies, but I don't like sweaters.
Figured it out yet? When I do this with students, I have to limit it to 5 minutes or so, or they will try to spend ALL hour figuring out the puzzle. After a few minutes, I insist that we have to go on with the lesson of the day. But, I will occasionally throw an "I like..but I don't like..." sentence into the lesson just to frustrate them.
The look of joy when one student figures it out and starts contributing their own sentences to the class is priceless. If students are really struggling to figure out the brain teaser, you can suggest that they record a list of things you like and don't like. Seeing the words written out should make the rule a bit more obvious.
We actually worked through this activity in one of my grad school classes. This could be a good introduction to teaching problem solving skills. Or, it's just a fun way to fill a few leftover minutes at the end of class.