I'm surprised "there" are people who don't understand this minor detail and yet are critical of others :)

"The trio of their, there, and they’re can flummox writers at all levels....Even though they sound the same, writing them down correctly is a different story! Let’s explore the differences between the three.

Their is the possessive case of the pronoun they, as in “They left their cell phones at home.” Their is generally plural, but is increasingly accepted in place of the singular his or her after indefinite singular antecedent, such as someone: “Someone left their book on the table.”

There is an adverb that means “in or at that place,” as in “She is there now.” In this sense, there is essentially the opposite of here. There is also used as a pronoun introducing a sentence or clause, as in “There is still hope.”

They’re is a contraction of the words they and are, as in “They’re mastering the differences between three homophones!”

http://www.dictionary.com/e/their-there-theyre/