As radiation cannot be detected by human senses, sources of radiation are required to be marked appropriately. Radioactive sources should be marked with the characteristic trefoil symbol and have the work "Radioactive" engraved on them. Any device that uses radioactive sources or containers that transport radioactive sources will also have this trefoil symbol and will be clearly marked with the radioactive isotope they contain. For radioactive devices specifically they are often brightly coloured to indicate they contain radioactive materials. For transportation devices, they are often very heavy for their size as they contain dense shielding materials such as lead or tungsten.