Is it "Thats why!" or "That's why!"? [closed]

I see it spelled both ways, what is the right way ?

So far I lived by the rule that apostrophe s means possession and without the apostrophe it is a simple abbreviation. Or is it more complicated ? (Sorry, non-native speaker.)

Solution 1:

There is no word "thats" in English.

"That's why" is a contraction of "that is why".

The apostrophe is used for both possession and contraction, but for the pronoun that it would be very unusual to use the possessive.

An exception is the word it, where it's is a contraction of "it is", and its is possessive (following the grammar of he/his, she/hers).