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.

Second, the brain quickly erases unimportant things from working memory to make room for new information. So unless those short-term memories are transferred into long-term memories (a process called consolidation), they're soon gone from conscious thought.

Because the brain isn't actually capable of multitasking, Miller said, it has to "juggle" different thoughts as our working memory darts around to different ideas. That requires conscious effort and attention, which are overseen by the brain's prefrontal cortex, a region involved with complex learning, decision making and reasoning. If attention becomes focused on only one of those thoughts or is diverted somewhere new, the brain loses track of the earlier thoughts.

"It drops one of the 'balls,' and that's why you forget stuff," Miller said.

The brain is especially likely to "drop the ball" from working memory when it's sleepy or impaired by alcohol or other drugs. Age is also a factor; Miller said working memory function peaks in a person's 20s and starts to decline during middle age.

But for those who regularly struggle with thoughts slipping their mind, Jaeggi and Miller have some evidence-based advice.

— What happens in our brains when we 'hear' our own thoughts?

— Why are kids such fast learners?

— What is consciousness?

To stop forgetting so many things in the first place, Miller advised against multitasking. "When you think you're multitasking, what you're doing instead is, you're juggling," he said, and juggling makes forgetting more likely.

Jaeggi gave a tip for what to do when a thought is already gone.

"Recreating the context can help," she said. That means going back into the room you were before, or retracing your thoughts. Those context clues might give the brain the extra boost it needs to reach back a few seconds in working memory and retrieve the thought before it's gone entirely.