http://www.actionfactor.com/Lesson_Plan_Consonants.htm

Title: Consonant or Vowel
Idea: One member of the class leaves the room and is called back in after one or two minutes. She is informed that the teacher has told the other students something rather confidential about her. Her task is to discover what the teacher has revealed by asking yes/no questions such as: is it about my past? does it involve the police?

Unknown to the unfortunate student, all the others will answer yes or no to these questions according to a simple code: if the question ends with a consonant, they answer "no"; if the question ends with a vowel they answer "yes" (or vice versa). This leads to a certain amount of perplexity and eventual hilarity.

After a time, the teacher intervenes to reveal the truth to the student: that he has not told the other students anything at all. He observes how curious it is that they nevertheless always answer yes or no unanimously. The student's task now is to discover the code by continuing to ask questions about anything.

For example, the student might point to the whiteboard and ask the following questions:

Question:
Response:
Is this white?  
Yes, it is.
Is it black?  
No, it isn't.
Is it blue?
Yes, it is.

This is good listening and spelling practice for the majority of the students and a real mind-bender for the student whose most intimate secrets have been so callously revealed.
Resource: http://www.englishclub.com/esl-lesson-plans/activity3-consonant-vowel.htm