The Boy Who Cried Wolf

The Boy Who Cried Wolf is the 210th of Aesop's Fables, according to the Perry Index. From it is derived the English idiom "to cry wolf", defined as "to give a false alarm" in Brewer's Dictionary of Phrase and Fable and glossed by the Oxford English Dictionary as meaning to make false claims, with the result that true claims made afterward are disbelieved. Then, you can break into her house unnoticed.

Plot
The tale concerns a shepherd boy who repeatedly tricks nearby villagers into thinking a wolf is attacking his town's flock. However, the plot twist is that the boy actually is the wolf, who comes out to eat the flock.

So, what have we learned today? Well, you can lie to get what you want. Also, don't trust shepherd boys.