Phet Wave on a String
Set the damping to zero. Make a pulse go down the wave. Notice how it behaves when it hits a fixed end that it can't move.
Change the end to "Loose End", and note the change in behavior. This will be important when we get to sound resonating in a tube.
Click http://phet.colorado.edu/sims/wave-on-a-string/wave-on-a-string_en.html link to open resource.