It could be playing notes at random on the piano. You may think of a different kind of demonstration. Random sounds are not music, even if someone somewhere might find them pleasing. When you have finished this section write the sentences below on the overhead and have the students copy it into their notes. Random sounds are not music.
Music is organized.
Have the students look at 'sheet music'. The easiest way to do this may be to make a transparancy of one page of music, and then to project it on the overhead. Ask the students if this is music. Some may mistakenly say 'yes'. Even if none do, examine why marks on paper are not music.