It's one of life's most frustrating annoyances: when a word gets stuck on the tip of your tongue, and try as you might, you just can't think of that actor's name or that particular word. According to SciShow, this happens once a week for most of us, and here's how it happens.
According to the video, learning new information activates a series of neuron clusters in our brain. The new information travels through complex clusters (that process the meaning of the info) to simpler ones (that process the phonology or sound of the info).
A tip of the tongue state is what is what happens when the meaning clusters light up, but the sound clusters don't activate completely because the signal in your brain takes a detour instead of following the right path. That's why you can often describe characteristics of the word, but not the whole word itself.
For example, let's say you just can't think of that one actor guy. You know, the dude who was in Frasier? His name is on the tip of your tongue, and you can picture his face perfectly, meaning your brain's complex clusters are activated, but those simple clusters just aren't firing correctly, and instead of David Hyde Pierce, you think Hyde Park. Or Pierce Brosnan.
It actually makes things worse when people try to help you out, too, SciShow explains. They don't know how your brain is working to process the information, so their "help" can throw you off. What does help is focusing on the sounds that come to mind as you're trying to come up with the word.
For more detail, check out the video.