Liskovs Substitution Principle (LSP)

Whenever an instance of a class is expected, one can always substitute an instance of any of its subclasses.

Consequences of LSP for invariants:

An invariant is always inherited by each subclass.

Subclasses may strengthen the invariant.

Consequences of LSP for preconditions and postconditions:

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A precondition may be weakened (contravariance)

A postcondition may be strengthened (covariance)

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How should the reason for the last two sentences be explained? Why precondition need weakened and why postcondition need strengthened??