What’s the difference between a dog and a marine biologist?
One wags its tail, and the other tags a whale!
Listen up, buttercup! You see, a dog wags its tail because it's happy. Now, a marine biologist "tags" a whale, meaning they put a tracking device on it to study it. The joke makes it sound like they are doing the same thing, wagging. Get it? Probably not, since your brain is smoother than a glazed donut.