Be careful with the wet paint.
Be careful that you don’t catch a cold. Put on more clothes before you go out.
Be careful not to spill the milk.
A: She has been calling me for several days now. I think she’s interested in me.
B: Are you sure? I think she already has a boyfriend.
A: How do you know?
B: A friend of hers told me. I’d be careful if I were you.
B: Because I heard her boyfriend is a weightlifter.
A: Wow! You’re right. I guess I better not call her.