<P> For carry on luggage, the Transportation Security Administration restricts liquids to 3.4 fluid ounces (100 mL). Originally, the agency planned to restrict to 3 ounces but switched it to 3.4 oz / 100 mL after pressure from the European Union . </P> <P> Most students are given some level of introduction to the metric system in elementary, middle, and high school, but the emphasis varies . Instruction primarily centers on the concepts of powers of 10, the associated prefixes, and the conversion from one prefix form to another . Units of length, volume, and mass are typically introduced through comparison with ordinary objects . The metric system is not reinforced much outside the classroom due to lack of popular metric use . Consequently, while students may understand some of the concepts underlying the metric system, they do not necessarily have an intuitive sense of the value of the units . </P> <P> In science education, students predominantly use metric measures in experiments and equations, allowing those interested in the sciences to become more familiar with the metric system . Science classes are requisites for an undergraduate degree in the U.S. which ensures most educated people have some familiarity with the metric system . Within higher education, the metric system is universal for classes in the sciences (see below), except for exercises designed to illustrate complex conversions . Some non-science textbooks in the U.S. use only the metric system without giving conversions in order to promote metrication . </P> <P> There are no U.S. customary units for electric current, potential difference, or charge since these concepts were developed after the international adoption of metric in science . The metric units ampere, volt, ohm and coulomb are the only units used . The SI term hertz has replaced the term cycles per second as a unit of frequency . </P>

Why dont we use the metric system in the us