The ubiquitous barcodes serve as the basis for the following computer magic. The applet below is made to represent the standard barcode representation of the UPC-A codes. This is something that is commonly (and not quite correctly) referred to as the barcode. Do the following: pick up a product. It could be a bottle of Cola, an ice cream pack, or anything else with an UPC-A code and a barcode on it. Initially the applet displays 12 question marks. By clicking on them you can upgrade each to any decimal digit. Do this with 11 digits and leave one question mark. Press the "Check" button. The computer will announce the missing digit.