learn the rule: there / they're / their

  • there is an adverb, indicating place or position
    • There is your sock.
    • Your potato is over there.
  • they're is a contraction (combination) of they and are and means they are
    • Do you know what they're doing?
    • They're stupid.
  • their is a plural possessive adjective, indicating ownership by more than one person or something, where the owners have been previously mentioned or are easily identified
    • Their sock is missing.
    • That is not their potato.
  • The three are not interchangeable.
  • Getting it wrong makes you look stupid. And ugly.