The microchip, which is programmed with an identification number, is encased in a biocompatible glad container small enough to fit into a hypodermic needle.  Once an animal is injected with the chip, it can be identified throughout its life by this one-of-a-kind number anywhere in the United States.  A painless special scanner is passed over the animal and the microchip sends a radio signal through the skin back to the scanner to read the chip.  The chip number is then displayed in the scanner viewing window.  Microchips are especially helpful when a pets tag is worn and illegible.