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The straight face test is like a lie detector for humans. If a lie is so stupid you can't help but laugh, it automatically fails the test.
My mom said she didn't eat my brother's homework. I knew she was lying because she was grinning like a fool.
My teacher told us there was no homework. I knew she was full of crap because she was smirking like a madwoman.
My friend said he didn't cheat on the test. I knew he was lying because he was laughing like a maniac.