"Be careful" has the best of intentions, but it's actually quite confusing to a child.
Years ago, I knew a girl who went to Nicaragua for a year and thought that the name of her host family's little son was Cuidado, because that's what they said every time they called him. It became a great joke, of course, once she realized that cuidado actually means "be careful" in Spanish.
While that story still makes me chuckle, I think it is indicative of a sad and more serious tendency that parents have to warn children constantly of impending danger, even when it's not there.
The biggest problem with shouting "Be careful!" is that it's such a general term that most kids have no clue what an adult is talking about. It could mean, "Look out for the poison ivy on the ground beside you," or "There's a black bear charging up behind you," or "Those rocks are slippery and your sandals have no grip." As Backwoods Mama writes in an excellent blog post on this topic:
"When you tell a child 'Be Careful!' it usually doesn’t get the response you’re hoping for; either they’ll give you a look of confusion ('what’s there to be afraid of?'), ignore you ('this isn’t scary!') or start crying (something really bad is about to happen!)."
None of this is fair to a kid who's just trying to explore, burn off some energy, and have some outdoor fun. Another reaction it gets, particularly when used repeatedly over time, is fear of the world. It trains kids to feel nervous and cautious about activities that they should be normally capable of doing.
So, the next time you feel those words shaping in your mouth, take a moment to rethink what you're about to say. Follow the smart advice of Backwoods Mama, in the following short graphic: