Lake
The tsunamis during the old moons (many moons ago) were responsible for the world's lakes.
Where did lakes come from? They are the residue of tsunamis from many moons ago.
こ
みずうみ
None
"That's where lakes come from," you tell the 子 (こ).
This is just a stupid story to get this 子 to stop asking "why, why, why?" about everything. Earlier, the 子 asked "where do lakes come from?" and you had to come up with a crazy story to quiet them.