Sorry about the rather unsavory photo, yes its a poo bag on a string (ask a dog owner if you don't know what a poo bag is). The bag was just a convenient way to attach my mobile phone to a piece of string so that I could swing it round my head to demonstrate the Doppler effect. Before the lesson I downloaded an mp3 file of a single note to my phone (musical notes) which I played whilst whirling the phone around my head. The students were not impressed until the track changed and the Doppler effect seemed to go into overdrive as the single note changed into music. As if that bit of magic wasn't enough later on in the lesson I played a random note on a slide whistle then picked out the exact same note from a box of tuning forks. I think I am going to stage this from now on by marking the whistle and fork.