Themes are groups of LEGO sets that are centered around a common subject and often share a common design or storyline. This is the primary method to sort LEGO sets. Over the long history of LEGO, dozens of themes have been introduced or phased out. While earlier themes had a lifetime that spanned several decades and involved the introduction of related subthemes, most newer themes are only in production for one to three years. One notable exception is BIONICLE, which lasted for ten years.