Complex sentence: This will be the first ever World Series in which both teams will be expansion teams, which are teams that were formed after the 1960 season; the New York Mets began play in 1962, while the Kansas City Royals began play in 1969.
Split into simpler sentences: This will be the first ever World Series in which both teams will be expansion teams, which are teams that were formed after the 1960 season. The New York Mets began play in 1962, while the Kansas City Royals began play in 1969.