Most hobbyists develop their skills by building kits, then progress to kitbashing, where various kits are combined to create a unique model before attempting to scratchbuild. Sometimes scratchbuilders utilize discarded parts of other models or toys, with or without modification, either in order to speed up the building process or to allow the process to continue in spite of certain parts being difficult to make. Some companies sell parts that are of little use to anyone but scratchbuilders.