Joke?

Funny

Teacher: Do you smoke?
John: No.
Teacher: Do you drink?
John: No.
Teacher: Do you do drugs?
John: No.
Teacher: Well then thats good. You dont have any bad habits.
John: Not really, I love to tell lies.

Not So Funny
If John loves to tell lies, he could be lying about not doing all the 3 bad habits and could therefore be doing them. But he could also be lying about loving to tell lies, in which case would mean hes telling the truth and not doing the 3 bad habits.

But then if what he said were true, then his last statement would be true and he would love to tell lies, and therefore be doing all the 3 bad habits. But then again, if what he said were true then he could be lying about lying and so on and so forth.

See how I can take a "joke" and turn it into a something like this lol