system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can I forget something that I was about to say?

Sometimes I want to get this one something out into the conversation but then poof its gone.
gpt
When you thought of what to say, your brain was using its "working memory." That's where you do your thinking. Sadly, it can only hold a handful of thoughts at a time. 

Every new thought you have has to take up space in your working memory. Every new perception is going to take up space in your working memory.

New thoughts and distractions can bump that thing you were about to say out of working memory/. And now it's gone.

Also, its signal decays over time, so if you don't keep thinking about it (and boosting its signal) it can die away and poof it's gone.