By catering to their adult fans with complex, detailed sets that often tap into 1980s and 1990s pop culture, Lego's new market full of fans 18 and older is growing more than ever before.

Graeme Dymond uses Duplo, the larger version of Lego made for toddlers, to make a Christmas display every year in his front yard, along with his own winter village. "Just about every Lego ...