Although many radioactive materials are silver-colored, metallic solids in their pure state, they can vary in color and exist in different physical states, including liquids and gases. They are also physically indistinguishable from other (nonradioactive) metals. In addition, ionizing radiation is not detectable by one’s senses. It cannot be seen, heard, smelled, tasted, or felt. For these reasons, simple visual inspection is insufficient to identify radioactive materials, and radiation sources can be virtually impossible to recognize without special markings.