44 TEACHING SCIENCE IN SECONDARY SCHOOLS

 b. Use historical approach by biographical sketches,
 such as Galileo, Archimedes, and Newton; by giving
 origin of physical principles, such as the origin of
 weights and measures.
 c. Take advantage of the student's background of ex-
 perience. Some students have had practical training
 in radio, garages, or construction work and can con-
 tribute greatly to class discussion. The effect of alti-
 tude on the boiling point may have been noticed by
 some student and others may have felt the variation
 of air pressure with altitude by "ears popping" going
 up in an elevator or in an airplane.
 d. Draw on practical applications in schools, sports,
 home, in the working world, and in amusement de-
 vices. One could use the following examples in teach-
 ing the value of levers: window pulleys for shades,
 baseball bats, scissors, wheel and axles, see-saws.
 e. Ask challenging questions; such as "Can iron float?"
 when introducing Archimedes' principle. Before dis-
 cussing latent heat, ask, "Why does heating a sub-
 stance not always change its temperature?" To
 arouse interest in the law of acceleration, ask, "Will
 a big rock and a little one fall at the same speed?"
 B. Introduce the metric system as an easy scientific tool.
 1. Show students that it is a system planned with a
 definite relationship between units of length, weight,
 and volume.
 2. Show it as an outgrowth of a system of tens.
 a. Other familiar systems are U. S. money, per-
 centage, centigrade thermometer, electrical pow-
 er unit, radio frequency and Olympic sports
 records.
 b. Demonstrate how convenient it is to use:
 (1) Show how easy it is to find area of card in
 metric system.
 (2) Compare changing one mile to inches with
 one kilometer to millimeters.