If they don't know, they are told friction did it. Students are asked to rub their hands, then really hard, and tell what happens. The teacher rotates the sound tubes so the students can hear and asks the class what makes the sound. Teacher asks students, grouped into cooperative teams, to list examples of friction and then to write a good definition. Answers are written on the board.
Class synthesizes answers into one good definition.
Students are led to understand that friction can be associated with solids, liquids and gasses and that there are subtle examples as well as flagrant examples. Teacher asks individual students to move a column of coins with one finger on the top of the column. Students test their lunar vehicle on various surfaces, i.e., wood, sandpaper, rug, ice and table top. Teacher asks groups to predict what will happen with the two bottles which she is about to let roll down an incline. Students will understand that water creates friction on the glass and slows down and stops that bottle's movement sooner than the empty bottle stops.