Why do We Get Tip of the Tongue Moments?

We've all experienced the tip of the tongue moment where we wanted to say something but just...  ...the possible cause of this phenomenon:To provoke tip -of-the- tongue moments, the researchers showed the bilinguals, a...  ...metronomes...

http://www.neatorama.com/2009/06/12/why-do-we-get-tip-of-the-tongue-moments/