(A customer buys an African clawed frog for her son. After bagging it, I bring it to the register. It swims around in the bag for a second as I set it down.)
Customer: “Oh my god, what is it doing?”
Me: “You mean swimming?”
Customer: “Why did it do that?”
Me: “It was probably coming up for a breath.”
Customer: “It breathes?”
Me: “Yes.”
Customer: “So, it’s not a fish?”
Related:
Something Smells Fishy

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(Customer comes in to buy some more mice because hers aren’t breeding.)
Customer: “How do you tell if the mice are male or female?”
Me: “Well, the easiest way to tell is the males have quite prominent testicles.”
Customer: “I didn’t know mice had testicles!”
Me: “That’s probably why your mice aren’t breeding.”

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(An irate customer comes flying through the door and slams a soaking wet, dead hamster on the counter. I recognize her as a woman I had sold a hamster to an hour ago.)
Me: “Oh my goodness what happened?!”
Customer: “I took this hamster home and he didn’t even live 10 minutes!”
Me: “Why is he all wet?”
Customer: “From being in the aquarium! He swam for awhile, but then he just dropped dead!
Me: “I’m sorry, are you telling me you put him in an aquarium full of water?”
Customer: “You told me hamsters can live in aquariums! You said I didn’t need to buy a cage specifically for hamsters!”
Me: “Hamsters can live in an aquarium, but not in water! When you were picking him out didn’t you notice the others are in empty aquariums with bedding?”
Customer: “You told me he could live in an aquarium! I demand a new hamster! One that is alive!”
Me: “I really don’t think any of our animals are going to work out for you.”

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(A dad and his children are in my checkout line. I’m ringing up items for the family when I hear his children talking to each other.)
8-year old son: “Want to hear a joke?
5-year old son: “Yeah!”
8-year old son: “Knock knock.”
5-year old son: “Who’s there?”
8-year old son: “Merry!”
5-year old son: “Merry who?”
8-year old son: “Merry, it’s almost Christmas!”
(I laugh.)
Dad: *looking at me and laughing too* “Yeah, I was wondering where he was going with that.”

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Customer: “Hey, do you guys sell any extinct fish?”
(I pause to see if she is messing with me, but she is serious.)
Me: “No, sorry. They are really hard to come by.”

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