1) If you're a lemon, you're sour.
2) You're sour, If you're a lemon.
3) If you're not a lemon, you're not sour.
4) You're not sour, If you're not a lemon.
The only statements that are true are 1 and 2. 3 and 4 are not true because if you're not a lemon, you could still be sour and you could still be sour if you're not a lemon.