Because its undersurface is covered with thousands of ceramic tiles which have a melting point of over 3000 K. They are so poor at conducting heat that their front surface can remain cold to the touch while their back surface a few inches away can be heated to over 1000 degrees for several minutes. Since reentry takes only a few minutes, the tiles can dissipate the heat they absorbed from the reentry friction without, apparently, much of it building up on the side nearest the shuttle's airframe. Some of the tiles receiving the greatest heat load may also partially melt. The tiles have to be replaced by fresh ones after each trip.