Why does it smell so bad?

This is not directly IB related but it is certainly Chemistry related. I was carrying out the reaction between iron filings and hydrochloric acid yesterday. An experiment I am sure many others out there have carried out. The context is the reactivity series and it also involved reacting magnesium, aluminium, copper and zinc with hydrochloric acid. I have carried out these reactions many times in many different schools and continents and always get the same puzzling result. The iron reacts (fine) butr ...