Watch it
Definition 1 of 1
Pro Tip 1/3Caution Alert
Used to quickly warn someone when they're potentially causing harm or inconvenience. 
Watch it! The floor is wet.
Pro Tip 2/3Tone Matters
The tone of voice can change meaning from friendly advice to a stern warning. 
Watch it, there's a pothole.
Pro Tip 3/3Not Literal
It doesn't mean to literally watch something, but to be mindful of your actions. 
Could you watch it with the noise? I'm studying.