Our brains receive input all day from all our senses. Some are necessary inputs and we concentrate on those and filter out others. Without this ability to filter out certain inputs, our minds would be very cluttered. Some people have difficulty filtering out unnecessary inputs. They are easily distracted, can't stay on task, and have a short attention span.
Some children seem to have a short fuse.
They just don't think before they act. Words come out of their mouth that they are sorry for even before they finish the sentence. They answer teacher's questions before the teacher's sentence is even finished. They get angry and yell, lash out, or hit because they don't pause to think before they act. ADHD is not a learning disability.