It's a common cause of Internet flame-wars, because in the UK "Lego" is used as an unpluralisable mass noun: "houses made of Lego" like "houses made of wood". In the US, to a large but not invariable extent, Lego is used as a common noun for a single piece: "houses made of Legos" like "houses made of logs". On reaching the official Internet flamewar age of about 15, most people have not yet encountered the other camp's usage, and so maintain that theirs is "correct" despite Lego not being invented in English in the first place (and in fact being a bilingual pun in Latin and Danish).

Peter