For Gingerbread houses, or graham cracker houses @ Christmas time, as is one of our many, many holiday traditions.
3 egg whites,
a splash of warm water
about 4-4 1/2 cups of powdered sugar
Mix really, really well with electric mixer on low speed. Might have to add more water or sugar to get the right consistency. For kids' crafting purposes, the right consistency is: thin enough to squeeze through a ziplock bag w/ the tip cut, for piping, but thick enough that it works like glue for candies onto the graham cracker houses.