Liskov Substitution Principle
It formally states that
-the preconditions of a subclass’s behaviour (methods) cannot be strengthened, meaning (among other things) that a sub-method cannot accept a narrower range of inputs than the original method.
-The post conditions of a sub-method cannot be weakened, meaning that the sub-method cannot have a broader range of effect than the original method.
WEAKER ==== WIDER; STRONGER ==== NARROWER.
For the LSP to be preserved, the sub-method should be smiling! (we don’t care about the super method, because it is what it is)