Social Studies...off track

Today I attempted to teach a social studies lesson about landforms that went completely off track...as many lessons with 7 year-olds do. I thought some of you might enjoy the thought process, or lack their of, that took us from landforms all the way to poisonous gas in sewers. We had just talked about islands, peninsulas, rivers, and oceans when...

Me: "Streams and rivers usually flow toward an ocean. Is the ocean fresh water or salt water?"
Students: "Salty!"
Me: Animals that live in the ocean, salt water, don't travel up rivers into the fresh water because they can't live in fresh water for very long."
Student 1: "I heard about a shark that swam up a river one time."
Me: "Yes, I heard of that too. That would be really surprising to see a shark in a river."

Students laugh.

Student 2: "One time I heard about a hippo that swam out of a river, into an ocean, and onto shore."
Me: "Oh my!"
Student 3: "One time I heard about a killer whale that took a woman under water and drowned her."
Me: "Yes, that did happen, but that whale was at Sea World, not out in the ocean."
Student 4: "Near my grandparents house there are these drains and there's a whole family of poisonous snakes that live down there with really red eyes."
Student 5: "You can't go down into sewers because there are poisonous gasses down there that kill people."
Student 6: "The other day, I saw Eli (another student in our class) at Target and I got to buy a shark movie."

Now, I promise that I'm a good teacher. This conversation took in total about 3 minutes and I let it keep going because they were enjoying it so much and we had already finished our social studies reading. I actually had a hard time keeping a straight face this time. I have no idea how they got from one thought to the other, but they did! This, is why I love teaching and why I don't mind coming to work every day. Happy hump day!