Think of two true things about yourself and one lie.
(For example: I got a scuba diving license when I was 15, I have been to Tanzania, My parents own a pizza restaurant)
Model the exercise first.
Students must ask you questions to find out which is the lie. (For example: Where did you get certified? Where did you last go diving? What is the capital of Tanzania? Etc.)
You can limit the number of questions or put a time limit on asking questions or simple have them ask questions until they think they know the lie. Finally the students then make their guesses and you tell them if they are right or not.
Now move on to letting your students try it out. You can put them into pairs or groups and have them take turns questioning each other.
You can make it into a game where people compete to find the most lies or who is the best liar.