One of the more wonderful things I have learned in some time:

[Exodus] says: “and the frog arose and covered the land of Egypt.” The word “frog” is written in the singular (“tzefardei’a” in the original Hebrew). What could the Torah be telling us? Rashi finds a Midrash that sees into the unusual wording of our verse: “There was one frog, and when they would hit it, it would spew out bands and bands of little frogs” (based on Midrash Tanchuma, Va’eira 14). Apparently, G-d caused one giant frog to emerge from the Nile. When the terrified Egyptians hit it, it spewed out smaller frogs. The more they struck it, the more frogs would come out. The more frogs came out, the more they would strike it. 

 There's a moral too. Happy Passover/Holy Week/Amphibian Appreciation!