Asarah B'Teves

This past Sunday, the 10th day of Teves, adults fasted, and have been doing so since the first Beis Hamikdosh was destroyed. It is not a happy day for us,  but rather, a day to remember how the destruction of the Beis Hamikdosh began. The Babylonians surrounded Yerushalayim and did not allow any one to enter or leave the city, get food or supplies from outside or send food or supplies to Jews on the inside even when everything was  depleted. The Jewish people, including their children, were hungry and thirsty. Thirty months later on the 17th day of Tammuz, the Babylonians torched the Beis Hamikdosh. 

The Jewish people weren't making proper choices for themselves and about each other. They weren't kind to one another for NO reason at all which made Hashem decide that He did not need a Beis Hamikdosh if people weren't treating each other kindly.

Hashem planted an idea in the Babylonian people's heads to destroy the Beis Hamikdosh, but He wanted them to do so slowly through a process which would give the Jewish people a chance to do Teshuva and start being nice truly. Hashem had the Babylonians surround Yerushalayim and since the Jews didn't do Teshuva, the Babylonians burned the Beis Hamikdosh on Tisha B'Av.

Now, every time we do a Mitzva we are bringing more light and goodness into the world and making it ready for the time when there will be true goodness when Moshiach comes.

In order for children to understand the concept of how we can bring Moshiach, we say that every time we do a Mitzva, we are adding another brick onto the Beis Hamikdosh and once it will be completed Moshiach will come.

http://www.chabad.org/library/article_cdo/aid/102698/jewish/10-Tevet.htm