The study of motion presents problems to students of beginning physics at all levels. Part of this problem arises because of students’ ‘‘common sense’’ ideas of motion. Six questions on trajectory motion, based on Galileo’s ‘‘thought experiments’’ in his D i a l o g u e C o n c e r n i n g t h e T w o C h i e f W o r l d S y s t e m s, were given to students in several different introductory physics classes. These responses are analyzed and discussed.