Complex sentence: Rat-baiting was a popular sport until the beginning of the 20th century; it involved filling a pit with several rats and then placing bets on how long it would take a terrier to kill them all.
Split into simpler sentences: Rat-baiting was a popular sport until the beginning of the 20th century. It involved filling a pit with several rats and then placing bets on how long it would take a terrier to kill them all.