Don't judge someone/something by its appearanc
1. She doesn't look very intelligent, but YOU CAN'T JUDGE A BOOK BY ITS COVER.
2. Don't go falling in love with that girl so fast. Wait until you get to know her better. YOU CAN'T JUDGE A BOOK BY ITS COVER.
3. That professor may seem slow and awkward, but DON'T JUDGE A BOOK BY ITS COVER. He is one of the brightest minds in his field.