FID tags do not have their own power supplies or batteries. Instead, the minute electrical current induced in the antenna by the incoming RF scan from the reader provides enough power for the tag to send a response. The received sig- nal charges an internal capacitor on the tag, which in turn supplies the power required to communicate with the reader. Because passive tags do not contain power supplies, they can be much smaller than active tags and have an unlimited life span. As of 2006, the smallest commercially available passive tags measured just 0.3 mm across and were thinner than a sheet of paper, making them just about invisible. Due to its power and cost, the response of a passive RFID tag is generally brief, typically just an ID number. Passive RFID tags can be read from a distance of about 20 feet (6 m). A semipassive RFID tag contains a small battery that boosts the range. Passive tags are generally read-only, SO the data they contain cannot be altered or written over. Some of the most common uses of passive RFID include animal identification, waste management, security and access control, work-in- process, asset tracking, and electronic commerce. The Chek Lap Kok airport in Hong Kong uses passive RFID tags to track the movement of every piece of luggage that passes through the baggage-handling system. Because passive tags are cheaper to manufacture than active tags, the majority of RFID tags in existence today are the passive type. As of 2006, passive tags, when bought in high volumes, cost an average of US$0.24 each. With volumes of 10 million units or more, the cost can drop to around US$0.07 per tag. The goal is to produce tags for less than US$0.05 to make widespread RFID tagging commercially viable. The main benefits of RFID include the fact that tags can be read from a dis- tance and from any orientation, SO they do not require line-of-sight conditions in order to be read. Read/write tags offer the additional benefit of allowing data to be changed dynamically at any time. Another benefit of RFID 