Have you ever walked into a room and suddenly forgotten why you went there? Or did you try to say something but blank on what you were going to say?

Our brains usually do a great job of handling lots of thoughts and actions, but sometimes they just seem to mess up. So, what happens when we forget what we were just thinking about?

To understand why we forget, we first need to know how our memory works and clear up some misunderstandings about it.

"Memory isn't just one thing," says Susanne Jaeggi, a psychology professor at Northeastern University. "There are different parts of memory that work together in different ways", he adds.

In this case, it's important to know about two types of memory: long-term memory and working memory. Long-term memory holds all the stuff we learn, experience, and skills we pick up over time—stuff that stays with us for hours or even our whole life. Working memory, though, is where our short-term thoughts live, but only for a few seconds or minutes at a time.

Think of working memory as a "sketchpad for your thoughts," says Earl K. Miller, a neuroscience professor at MIT. When you have new ideas, conversations, or sensory experiences, they go through your working memory. But it's also what makes us forget things quickly.

Here's why:

First, working memory has a very limited amount of space. Psychologists believe we can only hold about four to seven "chunks" of information in our working memory at once. These chunks could be things like numbers, words, or even short phrases. Instead of holding everything at once, our brain bounces from one thought to another, and sometimes a thought gets lost in the mix, says Miller.

Second, the brain is always clearing out unimportant things from working memory to make space for new information. So unless those short-term memories are moved into long-term memory (which is called "consolidation"), they vanish pretty quickly.

Our brains also don't multitask well. According to Miller, when we try to juggle too many thoughts at once, it's like trying to keep a bunch of balls in the air. If we focus on one thought too much, we drop the others. That's why we forget stuff!

The brain is more likely to "drop the ball" when we're tired, or distracted by other substances. As we get older, it becomes a little harder to juggle thoughts, too. Working memory works best when we're in our 20s, but it starts to slow down in middle age.

But don't worry—if you often forget things, there are ways to help! Miller advises to stop trying to multitask. "When you think you're multitasking, what you're really doing is juggling," he says, "and juggling makes forgetting more likely."

Jaeggi also has a tip for you when your thoughts already slipped away. If you can't remember something, try to recreate the situation where you first had that thought. Go back into the room or retrace your steps. Those little clues can help your brain find the thought again before it completely disappears.

So, the next time you forget what you were thinking, don't stress—it's just your brain doing its thing!