1. Do you really think you can fool me? I’m not that dumb. I wasn’t born yesterday , you know.
2. Philip is going to get hurt if he goes on trusting everyone the way he does. He never suspects that people make friends with him just because he is rich. He acts like he was born yesterday.

Compare to: wet behind the ears. The expression suggests that someone who was born yesterday has not learned to distrust or be suspicious of people and is therefore naive. Wet behind the ears suggests inexperience, while born yesterday suggests that the person is easily fooled.