Why do we... clink glasses?

In the world of Rome's Revolution, Rei is always saying or doing something that Rome has no clue what he means. Many of our colloquial expressions and actions require a cultural context to make any sense at all.

There is a little scene near the end of Rome's Revolution which showed how strange this custom appeared to the Vuduri:

     “Well, I am glad you came,” Rei said. “I would like to make a toast.”
     “What is a toast?” Rome asked.
     “It is just for good wishes. Watch.” Rei raised up his water glass. The others raised their water glasses, not really knowing what to do. “Here is to no more adventure for a while!”
     “Now what?” Rome asked.
     “You clink the glasses together for good luck,” To demonstrate, Rei touched his glass first to Rome’s then to Fridone’s and then to Binoda’s.
     “That is a strange custom,” Binoda said. “What is its purpose?”
     “I do not really know,” said Rei. “But it is fun.”
     “Let me try it,” Rome said. “Here is to the end of the Onsiras’ threat and peace for all.” She and the others clinked their glasses together.

So this leads to today's question is: Why do we clink glasses during a toast?

I originally thought its purpose was so that liquid from your glass would slosh into the glass of the other person's and by intermixing the two liquids, you could prove that you weren't trying to poison the other person. Turns out, that's not so practical. Your glass would have to be filled to the brim and likely would splash a lot of wine/mead/whatever to the floor.

No, clinking glasses is related to the origin of making a toast in the first place. A king or chief would put a piece of spiced bread in a communal bowl and then everyone would drink from the same bowl to show good cheer. Once people started drinking from their own glasses, that sense of community got lost. They still had the toast but no big honkin' bowl. So clinking glasses is a modern form of showing that we are all spiritually drinking from the same bowl, enforcing the spirit of camaraderie that was lost when people moved toward hygiene.

Tomorrow, why do we knock on wood?