It'sIt's is short for it is or it has. This is a 100% rule. It cannot be used for anything else. If you cannot expand it's toit is or it has, then it is wrong.
ItsIts is like his and her. Possessive.
His is used for masculine things. (These are his pies.)
Her is used for feminine things. (These are her flowers.)
Its is used for neuter things. (These are its footprints.)