All three have troughs and crests. Sound is a disturbance of air waves with pitch. Water waves travel in a circle away from the source of disturbance.
Light waves travel in a straight line unless they meet an obstacle.
The purpose of this lesson is to give children a basic understanding of the physical properties of waves without becoming too technical. They are to use a hands-on method using toys to become aware that waves are in air, water and light. A demonstration is made with a slinky. Ask children to define the part they think of as a crest and that part which is a trough. Draw a simple diagram on the board, labeling each.