Examples of nouns that function as both singular and plural are:aircraftantelopebarracksbisoncrossroadsdeerheadquartersmooseoffspringsalmonseriessheepshrimpswinetroutThose kinds of words are called, "Plurale Tantum" (which in Latin, means, "only a plural," for some reason). There are scores of them. If you Google your question, you will find a link to a site that promises 101 of them. Plurale Tantum (hmmm. I wonder what the plural of that is). I tried to attach a link, but that is not permitted on Answers.com. You may have to do the search yourself.