Parables are true or realistic stories that illustrate a deep spiritual truth not previously understood by man. Parables clearly divide between the story part and the spiritual lesson being taught. A short saying to be taken literally itself to teach some obvious human wisdom. Parables are longer, more illustrative and teach a hidden truth. An allegory transfers the properties of one thing to another.
Parables compare two separate things to one another.
Thus parables sifted the audience and found out the willing hearers.