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 also produces a very smelly gas – I suspect it is hydrogen sulfide – but if it is, where does the sulfur come from. Is it simply that the iron contains sulfur impurities or is there a more sinister reaction at work?

Answers (if you know or just if you think you know) below please!