My wife and I pay 10 percent of our income to the LDS Church as "tithing". A 10 percent tithe is taught in the Bible and is a principle Mormons are asked to adhere to.
We believe that by giving to the Lord, we show the faith that He'll provide a way for us to have the shelter, food, clothing, and other things we need. During my full-time mission in Argentina I often saw this to be true. The country was experiencing a period of great financial devastation and unemployment was through the roof. I knew several Church members who had to personally live the following scripture, putting the Lord to the test:
"Bring ye all the tithes into the storehouse, that there may be meat in mine house, and prove me now herewith, saith the LORD of hosts, if I will not open you the windows of heaven, and pour you out a blessing, that there shall not be room enough to receive it." (Malachi 3:10)
I was amazed in particular about how one old man, a Chilean immigrant, always seemed to find odd jobs to put food on the table. He was well past his prime years for manual labor and had limited education. However, I saw that he paid his tithing and did his best to serve in the Church however he was asked. His faith opened the "windows of heaven" and although his circumstances were humble, they were always adequate for his needs.
I want my children to understand why we pay tithing, so tonight for Family Home Evening we endeavored to teach our three year old son about it. We gave him ten pennies. We explained that this represented the blessings that Heavenly Father gives us. For each coin we gave him, he cheerfully replied "Thank you!"
We then asked him to return one of the pennies. He was happy to do so knowing that he could keep the other nine.
We then repeated this exercise with dimes and with quarters. Although I don't expect our son to know fractions at his age, hopefully he got something of the "one out of ten" principle.
In the end, we had 36 cents of tithing. He enjoyed putting it in the envelope and watching me fill out the donation slip. We showed him the Bishop's name on the envelope and explained to our son that he can give the envelope to the Bishop on Sunday.
After the lesson we had to run an errand, so at the store our son used his money to buy a treat. He picked "Dots" and we shared a few before bed (and before brushing teeth).
God's blessings are indeed sweet.
To read a Mormon church leader's talk on tithing, which includes some information about how the money is used, see Elder Robert D. Hales' Tithing: A Test of Faith with Eternal Blessings.