http://revach.net/article.php?id=3639

Section:  Avodah   Category: 48 Kinyanei Torah
Kinyan 3 Arichas Sefasayim - Get The Word Out
"Oib Es Felt In Hasbara Es Felt In Havana".  This famous quote, often attributed to Rav Chaim Volozhin, tells us that something may seem very clear in your mind but when you try to articulate it suddenly things break down.  Verbalizing your thoughts helps crystalize them and it is a good litmus test of true understanding.

Interestingly, when you count money or if you've watched other people do so, you may have noticed that they count out loud.  Not that counting is that complicated but when it comes to precious money no one wants to make a mistake, and articulating the numbers is going to give you more accurate results than by counting in the recesses of your mind.  Similarly showing gratitude or remorse in a variety of ways often doesn't do the job until you have said the magic words like "Thank you" or "I'm sorry".  Only by hearing this sentiment articulated from your lips does the other person feel that your appreciation or regret is clear in your mind.  Sadly even when it comes to Lashon Hara, two people can both be thinking the same thing about the same person for a very obvious reason, yet their Yetzer hara is not appeased until the gossip has escaped their lips in all the gory detail.

Torah is complex.  We can build complex structures in our mind.  But just like mere thoughts, we need to express them verbally, to test them if they are truly coherent.  If they pass this minimal test then they have a better chance of standing the test of time and remaining with you longer than a fleeting thought.