Chopping up onions activates a chemical reaction that releases sulfur gas. Water in the human eye reacts with sulfur gas to create sulfuric acid. Although the amount of acid is very small, it's enough to cause a burning sensation in the eye.
Fleas can jump more than 200 times their body length, which totals a maximum of 7 inches. If humans could jump as far as a flea relative to their body size, some would be able to jump more than 1200 feet (360 meters) at a time.