שמות ד ,ב : וּבְכָה וּבְעַמְךָ וּבְכָל־עֲבָדֶיךָ יַעֲלוּ הַצְפַרְ דְ עִים:
The Baal haTurim observes the word ובכה appears two other times in Tanach besides this time. Another time it appears in Dovid’s song of deliverance (Shmuel II 22,30) כי בכה ארוץ גדוד (for with You I will run towards a troop. The third time is in Tehillim 141,8 בכה חסיתי (I have sought refuge in You).
This teaches that the frogs swarmed through Mitzrayim in waves of troops. Further, the plague of the frogs did not affect the Jews since they sought refuge in HaShem.
Throughout this plague, the Torah uses the term צפרדעים which is plural, a total of 10 times. However, at the onset of the plague, the word used is צפרדע in singular. This implies that initially only one frog emerged from the river, and then the Egyptians struck it and swarms of frogs streamed forth from that first frog. According to another opinion, the first frog emerged from the river and crocked very loudly to summon all the frogs of the world. These frogs swarmed into Mitzrayim from all directions.
