Futility Closet gives us a crime scene and a logic puzzle to solve it. Five children go into a candy store, and one of them steals a box of candy. Let's assume that none of them have chocolate on their faces, and the stolen box is not in their possession at the moment. Each of the five children gives a statement of three sentences.
Ivan:
1. I didn’t take the box of candy.
2. I have never stolen anything.
3. Dennis did it.
Sylvia:
4. I didn’t take the box of candy.
5. I’m rich and I can buy my own candy.
6. Linda knows who the crook is.
Ernie:
7. I didn’t take the box of candy.
8. I didn’t know Linda until this year.
9. Dennis did it.
Dennis:
10. I didn’t take the box of candy.
11. Linda did it.
12. Ivan is lying when he says I stole the candy.
Linda:
13. I didn’t take the box of candy.
14. Sylvia is guilty.
15. Ernie can vouch for me, because he has known me since I was a baby eight years ago.
Okay, the clue is that each child told the truth in two sentences and lied in one sentence. Who stole the candy? Don't let the fact that there are 15 sentences deter you; it's not that difficult when you get into it. When you come to an answer or give up, see the explanation at Futility Closet. -via Boing Boing
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Ivan's first two statements are either both true or both false. Given the rules, both must be true and the third statement is false. Dennis is not the thief.
Jumping to Ernie. His third statement is the same as Ivan's and must be false. Therefore his other two are true.
Look at Linda's third statement. It contradicts Ernie's second and must be false and the rest of her statements are true. Therefore Sylvia stole the candy.