It’s a difficult thing to ponder—unless you ponder its difficulty.
Its [THINK: yours (no apostrophe)]
It’s [THINK: what’s (apostrophe)]
“Its is the possessive form of it…many possessive forms have an apostrophe…but the possessive its is a pronoun, and, like other possessive pronouns (his, hers, yours, and theirs), is written without that particular bit of punctuation…
It’s is a contraction of the words it and is, just as what’s, how’s, and she’s are contractions of what is, how is, and she is….”
[read more about it on blog.dictionary.com]