Why do we forget things we were just thinking about?

Have you ever walked into a room and forgotten why you went in there, or been about to speak but suddenly realized you had no idea what you were going to say? The human brain normally balances countless inputs, thoughts and actions, but sometimes, it seems to short-circuit. So what really happens when we forget what we were just thinking about?

Understanding why we forget first requires an understanding of how our memory works — and dispelling some myths about memory.

"Memory is not just one thing," Susanne Jaeggi, a professor of psychology at Northeastern University, told Live Science. "There are very different components of memory, and they're also related to different cognitive processes."

In this case, it's important to know two different types of memory: long-term and working memory. Long-term memories are a broad, multifaceted category of memories that involve knowledge, experiences and skills stored in the brain for extended periods — from hours up to an entire lifetime. On the other hand, thoughts in working memory dash through the mind for only seconds or minutes at a time.

Related: 'Short-term memory illusions' can warp human recollections just seconds after events, study suggests

Working memory is like the "sketchpad of conscious thought," Earl K. Miller, a professor of neuroScience at MIT, told Live Science. Every tidbit of new information, inner dialogue and sensory input routes through working memory, and certain characteristics of working memory likely explain why we forget those thoughts.

First, working memory has very limited capacity. There's been some debate over exactly what the limit is and how to test for it, but psychologists estimate that people can hold only about four to seven "chunks" of information — such as letters, digits, words or phrases — in their working memory at a time. Rather than being aware of all of these "chunks" simultaneously, the brain bounces around from one idea to another, making it more likely that one gets lost in the shuffle, Miller explained.