German is famous for its compounds. I'm not sure this is fair, really, because as far as I can tell it just doesn't write the spaces (I'm being facetious - but there is a conceptual issue here). No matter. It is famous for them. I said to my first years the other day that while English can't have plurals within compounds (eg it's toothbrush, not teethbrush, even though it's a brush for your teeth), German can. And this is true: bookshelf in German is Bücherregal, literally booksshelf. But it's not that simple. This is language we're talking about, after all.
German and English can both have irregular plurals in compounds, as well as singulars. English can have teethmarks, for instance (although personally I would prefer toothmarks). But neither language can have regular plurals in compounds, so we have mice eater but not rats eater and Bücherregal but not Autosberg ('cars heap'). (Links go to articles testing this idea.) The difference is that German has many more irregular plurals: their default (=regular) plural is the -s suffix that I mentioned in my Euros post. All the others, which put together are way more common, are in effect irregular.