Ever wonder about the Mormon's magic underwear? Here is it explained. From MormonUnderwear.com
To millions of Mormons around the world, garments are a special piece of clothing worn as a symbolic gesture of the promises that they have made to God. The garment is always worn under other clothing, next to the skin. For most people who wear it, the garment takes the place of regular underwear. (For this reason, some people refer to the garment as Mormon undergarments or as Mormon underwear. Most LDS prefer simply using the term "garment" instead.)
The garment is directly related to Mormon temples. It is there that faithful members first receive the garment after individual instruction on what it stands for and how it should be cared for. The garment is worn as part of a special ceremony called the temple endowment. The garment is worn at all times (day and night) by members as a constant reminder of the promises they have made to God in the Temple to be honest and faithful to the commandments of the Lord.
Here is an amusing video where John Safran, from the Aussie TV program John Safran v God talks to people on the street about Mormon magic underpants and he actually goes to a Mormon store and buys some and tries them on! Too funny.
Construction and symbolism of the garment