Players start the game with the ability to equip two weapons but later gain up to four weapon slots, as well as slots for an energy shield, a grenade modification, and a class modification. Items collected can be sold back at vendors for money that then can be used to buy better items. One of the key features of "Borderlands" is the randomly generated weapons and items created either as dropped by enemies, found in storage chests about the game, on the ground, sold at vendors in the game, or as quest reward items. The game uses a "Procedural Content Creation System" to create these weapons and items, which can alter their firepower, rate of fire, and accuracy, add in elemental effects such as a chance to set foes on fire or cover them in burning acid, and at rare times other special bonuses such as regenerating the player's ammo. A color-coded scale is used to indicate the rarity of the weapon or item. It was estimated that the random system could generate over 17 million variations of weapons, but actually only resulted in a little over 3,500,000. The Procedural system is also used to create the characteristic of random enemies that the player may face. This allows for enemies of the same species to have widely varying attacks: for example, variations of "spiderants" in the game could leap around and would jump onto players' faces, while another variant can roll up into a ball and attack people, depending on the content generator.