    *  none
    * aluminum
    * brass
    * brick
    * cardboard
    * cement
    * ceramictile
    * clay
    * copper
    * dirt
    * glass
    * grass
    * gravel
    * hardwood
    * ice
    * iron
    * linoleum
    * mud
    * pavement
    * plank
    * plaster
    * plastic
    * rock
    * rug
    * sand
    * softwood
    * snow
    * steel
    * stucco
    * tin
    * titanium
    * water
