Containers, vessels, enclosures, or other fluid system are sometimes tested for leaks - to see if there is any leakage and to find where the leaks are so corrective action can be taken. There are several methods for leak testing, depending on the situation. Sometimes leakage of fluid may make a sound which can be detected. Tires, engine radiators, and maybe some other smaller vessels may be tested by pressurizing them with air and submerging them in water to see where air bubbles come out to indicate a leak. If submerging in water is not possible, then pressurization with air followed by covering the area to be tested with a soap solution is done to see if soap bubbles form, which indicate a leak. Other types of testing for gas leaks may involve testing for the outleaking gases with sensors which can detect that gas, for example - special sensing instruments for detecting natural gas. U.S. federal safety law now requires natural gas companies to conduct testing for gas leaks upstream of their customer's gas meters. Where liquids are used, special color dyes may be added to help see the leakage. Other detectable substances in one of the liquids may be tested, such as saline to find a leak in a sea water system, or detectable substances may even be deliberately added to test for leakage.