GSM models of the iPhone use a SIM card to identify themselves to the GSM network. The SIM sits in a tray, which is inserted into a slot at the top of the device. The SIM tray can be ejected with a paper clip or the "SIM ejector tool" (a simple piece of die-cut sheet metal) included with the iPhone 3G and 3GS in the United States and with all models elsewhere in the world. Some iPhone models shipped with a SIM ejector tool which was fabricated from an alloy dubbed "Liquidmetal". In most countries, the iPhone is usually sold with a SIM lock, which prevents the iPhone from being used on a different mobile network.