Tritium, because it has an half-life.
You: What did the scientist say when he discovered tow isotopes of Helium? Your friend: what?
Friend : oh that's funny.
You: yeah, I know. It was a noble joke.
Friend: ugh. groans
You: actually, I expected no reaction at all.
Now it’s an isotop.
I really need to keep an ion them.
Q: what did the scientist say when they found 2 isotopes of helium?