A data type describes the kind of value a program expects and the operations that make sense for it. Common examples include text, numbers, and Boolean values representing true or false. A value’s appearance alone may not reveal its intended meaning. A phone number is often stored as text because leading zeros and a plus sign can matter, and arithmetic is not its purpose. A count of completed tasks is numeric. A completion flag is Boolean, although the application must define exactly what completion means. Converting between types can lose information or fail. A blank field is not always the same as zero, and missing data is not necessarily false. Good input validation checks the meaning and allowed range rather than merely accepting anything that looks similar. Practice: Choose suitable types for a postal code, temperature reading, and subscription-active flag. Explain one mistake that could result from choosing an unsuitable type.