My name is Barry Reitman. I'm the author of "Secrets, Tips and Tricks of a Powerful Memory," and I'd like to discuss the question of what happens when something is on the tip of your tongue. It happens to all of us. I'm going to divide this answer into two parts: One is for those things that you never properly memorized, things you never applied any mnemonic system, you never applied a system at all. It's just something you may have read sometime or a song you might have heard or a picture you may have seen. And what's happening is there's a lot of scientific literature about it, indeed. It's the TOT - tip of tongue syndrome - and you can find thousands and thousands of pages about it.
Technically what's happening, I can't tell you. But the bottom line what's happening is you didn't really know it well to begin with as a rule. What can you do about it? All kinds of theories, I can only suggest one. If something is on the tip of your tongue, and you can't spit it out, back away from it, relax a little bit.
Another theory that you're going to hear about is clench your left fist. Some scientists will say that what that does is stimulate some neurons that are going left wrist to the right side of the brain and all of that material that I'm not really sure about: I'm not a scientist. But I do think that the very act of clenching your fist takes your mind away of that terribly troubling sense of the tip of tongue syndrome and may relieve it. Give it a try. See what works for you.