Or thinking quietly in your head, 'thinking out loud' in your head, and actually vocalizing out loud. This is (partly?) learned behavior. Children under the ag

Or thinking quietly in your head, 'thinking out loud' in your head, and actually vocalizing out loud. This is (partly?) learned behavior. Children under the age of 5-6 will have audible self-directed speech, but eventually they learn to mute this and remove vocalization altogether.