We deal with a lot of students who say anything that pops into their heads, and it gets them into a lot of trouble. I n fact we deal with a lot of adults with the same issue (raise your hand if you have ever worked for The Man Without a Filter).
A lot of kids don't know that their thoughts are separate from their feelings and actions. The fancy term for this is "metacognition," which is "thinking about thinking." The "thought bubble" is a concept that can illustrate this idea.
Kids are familiar with the difference between thought bubbles and speech bubbles in comics or cartoons. But you can explain that a speech bubble contains words that are spoken aloud, while a thought bubble contains words, ideas, or pictures that are in someone's brain. No one else can see or hear them. For that reason, we are able to think anything we want---even if it's angry or mean---but if we say our thoughts out loud, sometimes we can get into trouble.
For a very intelligent second grader who kept blurting out his feelings and disrupting the class, I made up this graphic of how to decide if something should stay in his thought bubble: