1. The Battle of Paoli: On September 21, 1777, the Battle of Paoli, also known as the Paoli Massacre, occurred during the Revolutionary War. British forces under General Charles Grey attacked a Continental Army camp in Paoli, Pennsylvania, leaving over 50 American soldiers dead and many wounded.

2. The First American Zoo: The first American zoo opened on September 21, 1874, in Philadelphia, Pennsylvania. The Philadelphia Zoo was home to a collection of animals from around the world and was the first zoo to have an animal hospital.

3. The Johnstown Flood: On September 21, 1889, the Johnstown Flood occurred in Pennsylvania. After several days of heavy rain, the South Fork Dam broke, causing a massive flood that devastated the town of Johnstown. Over 2,200 people lost their lives and the flood remains one of the deadliest natural disasters in American history.

4. The First Keystone Kops Film: On September 21, 1912, the first Keystone Kops film, "Hoffmeyer's Legacy," was released. The Keystone Kops were a group of bumbling policemen portrayed in silent comedies by the Keystone Film Company, based in Southern California.

5. The Erie Canal Opens: On September 21, 1825, the Erie Canal opened in New York, providing a water route for goods from the Great Lakes to the Atlantic Ocean. This canal helped to spur economic growth in Pennsylvania and other surrounding states by promoting trade and transportation.