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.