In all games in the series except "Command & Conquer: Generals" and its expansion "Zero Hour", funds are acquired by specialised "harvester" units which bring their cargo (Tiberium for the "Tiberian" series of games or ore or the more valuable gems for the "Red Alert" series) to a "refinery" structure. This in turn will convert the raw material into usable resources, expressed as credits. The raw materials themselves, in games released before "Red Alert 2" as well as "Command & Conquer 3" require storage space in the form of refineries and, in the case of excess, "storage silo" structures. In "Generals" and "Zero Hour", funds are collected by two methods: collection of supplies by specialised units and converted to money in "supply centers" or directly produced by specialised units, buildings, or tech buildings at a set interval of time.