The Augean Stables
Herakles cleaning the Augean Stables
(Metope from Olympia, ca. 460 BC)
Hercules had to clean out the horse stables of King Augeus, which hadn't been cleaned in years and were all full of horse poop. It seemed like it would take forever, but Hercules found a river nearby and dammed it up so it ran right through the stables and washed them all clean. This picture of it, from a metope of the Temple of Zeus at Olympia (about 460 BC) shows Hercules digging, while Athena shows him what to do.
Learn by doing: check out some horses
More Labors of Hercules
Twelve Labors of Hercules (Step into Reading, Step 3), by Marc Cerasini. Very easy, for beginning readers.
The Story of Hercules (Dover Children's Thrift Classics), by Robert Blaisdell (1997). Easy reading, very cheap.
Hercules, by Nancy Loewen (1999). More sophisticated, with a look at how the myth was passed on and what it meant to people, as well as the story itself.
D'aulaire's Book of Greek Myths, by Edgar and Ingri D'Aulaire. (Look under Heracles).
The Myths of Herakles in Ancient Greece, by Mark W. Padilla (1998). By a specialist, for serious readers.